To encounter or try to communicate or work with someone at an inopportune time, such as when they are busy or preoccupied; to catch one off guard. I'll definitely help you with that report later—you just happened to catch me on the hop right now. If Dean was grumpy, you probably just caught him on the hop.

Caught unawares or in a state of unpreparedness. Please forgive my distracted attention—I'm afraid I was caught on the hop by your sudden arrival. Terry hadn't prepared anything to say at the reception, so he found himself caught on the hop when they asked him to say a few words.