Odd Focusing Issue - 90 APO Summicron

Several months ago, I purchased a 90 APO Summicron from KEH. It was listed as Excellent, and it was. I used it on several trips and liked to bring it along with my 35 Summilux for their unique looks.

It was tack sharp at every f stop. Then two weeks ago, I was in Toronto and suddenly it didn't have any clear focus at infinity except at f16. I thought that it had to be the M11's recent update, so I changed settings to automatic, then manual. Still no luck.  When I got home I mounted it on SL2 and still the same issue at every f stop.

Eventually I sent it to Leica USA in New Jersey. I just signed the work order to repair. It's just over $300 (USD).

I asked the technician what happened and he said he didn't know exactly, he said it needed "extreme front focusing alignment"

The problem came up very quickly. It worked great on a recent trip. Then suddenly, it didn't. The lens, when not being used, always has the caps on and is kept in a padded compartment of my bag. When it's mounted on my M11 or SL2, the cameras are always gently handled.  At home, it's kept on a shelf free from humidity.

I'm curious how front focus get out of alignment so severely so I can avoid this in the future.  Has anyone else had similar issues?  Are there ways for the not so faint of heart to do this kind of adjustment a home?

Thanks,

-Ted