Like Josh said...They're personality based.
I probably choose a personality not common to most, and I think the only HC in the game who had the same one is Lane Kiffin.
The personality I choose is "Analyst".
Through 5 games I've only had 2, AGAIN, TWO, defining moments as San Fran's HC.
Both came in a hm gm vs NE. One was the typical go for it on 4th and 1, except I was only down by 10 and was in FG range so I decided to go for the FG. That resulted in a hit to my approval, but I was willing to take it. Another one came in the 1st half where I was down by 10 again, and had the ball on the NE 27 on 4th and 2.
I went for it, was able to get into the redzone, and came w/in a FG of tying to finish the 1st half.
The defining moments made sense though. You know why...because NE opened the game up in the 2nd half, and ended up beating me 51-24.
It was 24-21 at the half.
W/ the Niners I couldn't play conservative w/ NE, but did and it backfired as I only got past the 30 once in the 2nd half.
NE opened the 2nd half w/ a kickoff return for a TD by Wes Welker, and it was ALL downhill from there.
I'll probably be a lot more aggressive in the future when it comes to defining moments.
Definitely personality plays a role. As a "Commander" personality w/ a failed ATL career I got quite a few defining moments a game. If you don't want to see as many maybe try the "Analyst" personality.