A change in selective pressure would trigger a change in the output of selection. The parent's point that reduction in trees would trigger a change to more walking behavior makes sense to me - the environment changed, which caused a change in selective pressures, thus making walking more favorable for survival and "triggering" the change.

A lot of that is tied up on how our language works. "The reason for this is" is ambiguous, because we use 'reason' to describe both purposeful decisions and coincidental explanations.

Have you ever tried to distinguish between "explaining" your behavior and "making excuses"? There is no clear difference -- or rather, people don't know what the difference is if there is one. It's not encoded in the language, but in our intent.

There is a lot of confusion about evolution because we simply don't have the words to make things less confusing. Trained scientists are better at it because they are better trained to see structure independent of language -- (I.e., to think logically instead of culturally.)

I think he is using the word more like a way of saying "the group of factors that ultimately led to this change" and not as if there was actually a change waiting for a trigger so that then "it can happen".

At least that's how I read it.

It was. The change was a shift in the tree population due to a latent factor; this latent factor is what resulted in walker-better-groups to outcompete tree-clinging-groups.

There is no anthropomorphizing here; it's simply semantics.