Adrenaline is a familiar friend that Stiles really wishes he'd see less of.
In the span of a few moments, he's been embarrassed, annoyed, paralyzed with fear, and then miraculously, saved, and relieved.
And if Derek asks, that's what Stiles is going to claim it is--his body's undue and potentially adverse reaction to stress and life-threatening situations.
He can feel Derek, his grip past snug and verging on painful, can feel the line of his body, hard, because even here, even in this world, Derek is 100% muscle. It must be his own adrenaline-fueled reaction that gives him the strength to turn and press his mouth against Derek's, the kiss hungry, if not outright demanding.