happen along/by (somewhere)
phrasal verb with happen verb [ I ] uk/ˈhæp.ən/ us/ˈhæp.ən/
mainly US
碰巧来到
to go to a place by chance or without planning to
I'd have drowned if he hadn't happened along and pulled me out of the river. 要不是他碰巧走过，把我从河里拉上来，我可能已经淹死了。