Composed a long comment here yesterday which I didn't post. Here's some ingredients in the mix:
With the treaty and Iran trying to re-integrate into the global economy it is insisting on its right to resume oil exportation to those (OPEC approved) levels that were in place before the sanctions. Saudi Arabia's flooding and depressing the global oil market hurt many petro-interests - the US shale oil/gas boom, Russia, and their arch-enemy Iran. Also hurt SA, but they will do what Wahhabists do.
and - Explorations in the central 'Stans suggest large oil and gas deposits. There is a proposed but not under construction pipeline route from Iran's northern border to Iran's coast - a relatively short (compared to the existing Russian territory pipeline network) and direct conduit to international markets for anticipated production from the central 'Stans. So long as Iran remains a sovereign and independent nation, with territorial control and the right to impose transportation royalty fees, the US will probably try to prevent this.
btw - speculation on these prospective fields in the 'Stans is quite far along and US interests hold at least 50% of the agreements for joint development. Exxon-Mobile is heavily involved.