My thinking is it’s uneven wear on the cassette , let me explain.
Having got pissed off with killing complete chain sets , every couple of months , I attempted the ‘3 chain drive train ‘
As in ,you fit a new chain set , then buy 2 additional chains. Theory is if you change the chains in rotation every 2 weeks ( or what ever ) the whole chain set will last much longer.
Well 2 weeks in , I changed the chain & guess what ? There was an annoying vibration , almost grinding noise & under load would skip on my 2 most used sprockets!
Conclusion , the tolerances are so fine on 10 speed kit , the slightest uneven wear can ruin your day.