In most situations like this, there is some excuse. Luke was in a huge hurry, about to miss a bus or needed to catch up to a friend, took a corner too fast and just ran flat into the unsuspecting pedestrian coming the other way. But this is Luke we are talking about, so no reasonable excuse is needed. He just wasn't minding his god damned business, either deep in thought or deeply thoughtless, and his natural carefreeness let him run smack into an oncoming ranger.

He could have kicked himself. Would, later. For now he bounces off the larger man's chest, backpedaling several feet, mildly disoriented, but instantly apologizing before he even recognizes who he's run into. "Sorry! Shit, um... I wasn't watching..."