I don’t think it had anything to do with the immigration bill, but I was on a thread where people actually seemed to believe that having all these Muslim doctors coming here was a good thing and Bush cancelled it out of annoyance at the failure of the immigration bill!
I remember reading about these visas when the offer was first made, and I thought it was a bad thing at the time. Obviously, the UK events have proved this to be the case. But I bet you’re right about some prior info all coming together after the bombings.