Seriously, what do the leaves have to do with anything? I was right with him, up to that point.
Only two things I could think is that either:
in that stand of trees, in the middle of campus, officer couldn't safely assess background safety when taking the shots...as in if he misses the truck, what's he gonna hit
or
During the chase, the leaves on the trees reduce the officer's sight lines and ability to assess the safety of continuing the chase at high speeds.
Just guesses, I thought the same thing...umm, is he missing the rest of that sentence?