I would say its just the time of year.

I work on a farm and have been bushhoggin' quite a bit this past week and I bump the deer out of their buffer zone right next to a bean field (This was PRIME bedding and fawning areas, and here I am shreddin it up, oh well.. bosses orders) and they just get out hope along about 30 yards and pay no attention to me.

Most of these are does, mind you so they have their little ones nearby, but deeer are so much more relaxed during the summer its crazy.

As far as using a tractor to your advantage, I say go for it. As soon as the deer become associated with an obect and learn that no dangerous consequences come of it, they will pay little attention to it until it threatens them, or they see it threaten another deer.

I would definitely give it a try trying to score on a deer, would be a lot of fun.