It may or may not be the case that they're disrupting shipping at the direction of Iran, but it almost certainly is the case that Iran equipped them, and that Iran can effectively instruct them to stop.
It's hard to make any kind of comment on a thread here without getting pulled towards one pole or another of the ongoing argument about what's happening in MENA right now; I'm doing my best to keep this as dry as possible, and to hedge statements by saying things like "the conventional geopolitical answer". You can find alternate geopolitical answers; in fact, you can probably find any answer you're looking for. All I can do is relay what I'm reading.