But what is his problem with the Iran deal anyway? Iran gave up practically all nuclear activity, what more does he want?
There is literally no reason to eliminate this deal unless the purpose is to harm Iran, either economically through new sanctions or military.
I highly suspect Israels lobby is pushing for eliminating the deal.
I'unno, trump apparently laundered money for one of iran's military/terrorist branches (forget the exact name, 'cause I've been awake for like five minutes -- pretty sure I've mentioned it waaaay upthread if you feel like searching for it). I could see harming iran in a bid to obfuscate criminal activities as being something trump decides to do.
Could also see someone on their side of things having fucked with his ego at some point during the process or since, which given trump's behavioral patterns would explain just about any level of hostility from him, up to and including inciting military conflict. That the deal happened under obama would qualify here on its own, really.
... 'course, now that my brain is working a bit more, most likely is just because his base lies out its fucking ass about and swallows any swill that shits on convenient brown people targets, of which for reasons various and historied iran happens to be. Trump'll take a dump on anything positive or even neutral about them for that reason alone, and the talking shitheels among the right wing have been mischaracterizing the iran deal to suck off racists and xenophobes since day 1 of negotiations.
Mind the other two are still decent possibility, as is stuff like one of the fucknut and/or corrupt as fuck individuals that he's surrounded himself with playing with trump's various character flaws to worsen relations with the country due to some grudge or bribe-giver's agenda or fucking spite etc., or whatever. There's basically a slate of possible and entirely too goddamn plausible causes for trump to be trying to incite hostilities with or undermine someone.