Maybe that PPP poll showing him trailing a Generic D by 7 points, combined with Rosen's announcement scared the shit out of Heller.
I've said it before, I think Murkowski is genuinely concerned about the Medicaid cuts to the point that she'll actually vote against it, Alaska in particular gets hit pretty hard by AHCA. I know that's lost on most Republican Senators but if a bill strongly adversely affects your state (even if it's a good bill on balance which AHCA definitely is not, it just hurts other states less) I'd expect any decent representative of their people to vote against it.
I know Democrats stand to gain plenty under an AHCA apocalypse but I'd really rather not. Plus it's easier for the next Dem president to improve ACA or transition to a universal system from there than fucking around cleaning up Trump's mess.