Bart is not good at keeping himself out of other people's business.
So maybe he's a little upset at Batman, the Batman, for not taking better care of his friend, of the former Robin, of Red Robin : a.k.a. Tim Drake. Bart's not the smartest of boys (smarter than you Kon, hah!) but he knows messed up when he sees it. Tim has been more of a hermit than usual since, well, a lot has happened. Their superboy is back from the dead, so is he, the Battle For the Cowl, the Batman being pulled back from god knows where by a loyal sidekick who is currently killing himself over cases and caffeine dependency spiced with severe lack of sleep.
So maybe marching all the way to the Justice League Watchtower was not in his best interest. It's for Tim, he holds onto the thought like a sword, a weapon to use against the adults who don't pay their sidekicks the respect they're earning right under their noses. For Tim.
And of course as soon as he spots the brooding, infamous Bat, Impulse deflates into an uncertain mess. "Uh."
(( out of chicken whizzes. hope this isn't too presumptuous??? i'm not sure how much of the comics you've read so... if you need another starter that is less specific or something lemme know ))