You'll encounter these guys roughly halfway through your playthrough. The bulldozer will sit in place, there will be animation of one of the guys jumping into it, before it slowly rolls forward.
They do not shoot, but take a ton of damage. At the beginning you'll only have your gun to deal with them. Get some distance and unload while they blunder towards you.
After killing the first boss (spider boss) you'll gete bombs, these will one shot the dozers if you get a direct hit. You lay them down with the Y button, and they have a red bar that fills up (takes about 1-2 seconds) before detonating.