Never mind, it just took over 15 minutes for the AI to decide on a course of action...
15 minutes on the AI is a bug - it should definitely be much faster than that. I think what is likely to be happening (given your screenshot - thanks!) is a combinatorial explosion in the number of possible targets and order of shooting. There is a supposed to be a check that simplifies the shooting possibilities in this situation, but it is either not working, or there are so many bases its not improving it enough.
Thankyou for reporting this. It is now logged in bug tracking and I will fix it in the next patch release.