The bumps were on the d and k keys because these letters are touched by the longest fingers, which come into contact with the keyboard first if the hands are relaxed.
Landing on the wrong key is recognised instantly.
Having to bend your three fingers on each hand, lifting up the thumbs to make room for letting the index fingers detect being in the right position takes more effort. Also leaning on the keyboard idling is keeping into contact with the middle fingers on the d and k keys, confirming correct placements while hand muscles are relaxed.
This may not be a big effort, but if it is one you have to sustain all day, it matters. Back in the day it was rumoured that Apple had its word processors tested by typists that were used to mechanical type writers. I can imagine they also had a voice in the decision of bumping the d and k.