I am guessing that 'starà' is the future form, since it is very similar to 'sarà', future of 'essere'. If I'm right, wouldn't this mean "What will be happening?" It seems like "What can be happening?" would need a different verb, maybe 'possere', if I have that infinitive right. I'm confused.
Stare is the right auxiliary to use with the gerund. Sta succedendo means It is happening and expresses continual action. The future stara' is apparently being used in a conjectural sense, i.e., What must be happening?, which is possible in Italian.
I knew that 'stare' was the right auxilliary, I just wondered why it wasn't translated in the future tense when 'starà' (which I hadn't seen before) seemed to be the future of 'stare', since it looked so much like 'sarà', which I had seen before. Thank you for your explanation that in Italian it can be used in this conjectural sense, too. :)
Maybe it's similar to how in colloquial English we could say, "That'll be him," when we hear a knock at the door and assume it is the expected person, while also expressing that we don't know for sure yet?