Another fundamental bit of context beyond the world of PC's is that video cameras were dropping in price, so people were much more likely to be doing video stuff even away from computers so there was suddenly a much larger market for the computer industry to tap into. America's funniest home videos launched in 1989, and wouldn't have been possible without practical portable camcorders being widespread. Likewise, the TV show COPS launched in 1989 because shooting TV on-location without needing a whole video truck was new and impressive, and in 1991 a citizen was able to document the Rodney King beating and it shocked the nation because it was so new for somebody to just have a handy camcorder when something was happening. Such attacks presumably also happened prior to 1991 -- there just weren't as many people with video cameras to document the phenomenon. (Things like portable 8mm film cameras did exist, but the practical differences between film and video are probably out of scope for a computing discussion.)
In 1985, the Amiga launched with fairly decent graphics by the standards of the time, and the ability to use NTSC compatible display timings and genlock so an off the shelf personal computer could be used as a part of TV production for the first time. Avid Media Composer launched in 1989, compatible with a 68020 Macintosh II from 1987, paired with dedicated video hardware. By 1990, the Amiga 2000 with a Video Toaster could do all sorts of live switching, keying, and cheesy digital video effects that used to require exotic dedicated hardware. Apple launched QuickTime in 1991. (I'll get back to that.) In the consumer space, video games were pushing ever more 'lifelike' and 'realistic' graphics as a selling point, kind of culminating in 1992 with Mortal Kombat directly using captured video frames as sprites to make the first video game characters that literally looked 'lifelike.'
So, at the start of the 1990's, you have a general emergence of video related technologies, and a general trend of general purpose personal computers being involved in some aspects of video production. There wasn't any single "lightbulb moment" in that path. Just a whole range of technologies becoming cheaper, more practical, and more desirable in a virtuous feedback loop.
The better the compression ratio of your video codec technology, the less hard drive space and performance you need. The simpler the decoder of your video codec, the less CPU power (or dedicated decoder chips) you need. If you have a video decoder in your display card, you don't need a ton of memory bandwidth between the CPU and the display hardware. If you have a lot of bandwidth, you can use simpler display hardware, etc. There's no one specific thing like, as soon as RAM hit X nanosecond timings, them boom it became possible to do video with that RAM. It was a balance of factors, where if any one technology in home computers had been worse, some other aspect of the system might have gotten more work to compensate.
Decoding a 640x480 24 bit RGB frame of video does require an exact minimum amount of RAM. But that doesn't mean video was impossible with less RAM -- it just meant people used crappier looking lower resolution videos, and videos got a bigger and better looking over time as memory and storage got cheaper. There was too much consumer interest, so nobody was going to throw their hands up and declare it impossible -- they just did they best they could with the constraints of the systems they were targeting at the time.
If I had to pick a single definitive technology that pushed video from exotic to ordinary, I'd say the launch of QuickTime in 1991. At that point you could write video production software while standing on the metaphorical shoulders of giants. You didn't need to implement your own file formats or codecs, and you didn't need to tie your application to specific hardware accelerators. Your customers could buy QuickTime compatible software today, with the reasonable expectation that the crappy video quality would get better over time when they ran your software in the future on better hardware and using better codecs. That meant that people making video could feel safe buying video software tools and not worry about the tool getting abandoned. That confidence in the marketplace resulted in fostering a market for video production related software, which led to a bunch of people being able to make video on their computers, which led to a bunch of people being able to make video for other people to play on their computers in FMV games and whatnot.
Now, QuickTime isn't really a technology, per se. It's just a standardized interface to some hardware and software technology that existed independently of QuickTime. Remember, AVID launched on Mac before QuickTime existed, so people were already working in that space - it absolutely wasn't a hard requirement. But the fact that a major platform vendor was treating video as a major selling point had a huge impact on the market and massively lowered the barriers to entry, so it's what I'd pick as the turning point. It was when everybody noticed that the technology was polished and shiny enough to start the gold rush.