There are already other articles claiming he was shot because he's Jewish and has supported Israel. Seems like the cops haven't arrested anybody yet, so we don't really know anything.
He was working on fusion technology, so you could just as well speculate it was fossil fuel interests involved, but that also seems purely speculative.
First thing that came to mind is that he might have been secretly working for Israel's nuclear program but this is all very speculative. It does feel plausible though given that Israel has already assassinated plenty of Iranian nuclear scientists; so there is some precedent for it.
There are lots of different scenarios you can conjure up. Another could be that Israel assassinated him after he refused to support their nuclear program OR to silence him.
He was working on fusion technology, so you could just as well speculate it was fossil fuel interests involved, but that also seems purely speculative.