a couple of ways I have seen this happen to me - sometimes I have caught it when looking back and fixed it, sometimes not.....
1. A player is moved to the starting slot but was set at mop up minutes - mop up setting overrides the start
2. A player is moved to the starting slot, but is already starting somewhere else. That cant be saved, but if you dont look and instead navigate away from the page the prior depth chart remains in force.
3. I click on the drop down of players for a slot and think I click on one guy but mistakenly click on the guy above or below him. Very easy to do if names are similar. Or if I am tired.
You can explore these possibilities by looking at who did start. Was it the depth chart from the prior game retained? then maybe option 1 happened. Did the guy who was in the second slot start? then maybe number 2. DId some random guy who appears next to the desired guy in the dropdown start? then maybe 3
BUT, if you looked at the depth chart the next day and the guy who was supposed to start but didnt was actually there (and wasnt set on mopup) then I have no answer