The symmetry of the bubble suggests a single high-energy source to me, most likely a black hole
My bet is on our supermassive central black hole. It seems likely that they erupt "quasar-like" in all galaxies that have them every once in a while. The burst of star formation is possible too, but strikes me as less likely.
Is it plausible for gravitational interaction within a massive cluster to cause it to behave collectively as a super star?
I don't know but my guess is probably not. Even in a densely packed globular custer, the stars are measurabler fractions of a light year apart.
And if the core density became high enough, could it collapse into a chain-reaction supernova?
Chain reaction supernovas might be possible in globular clusters -- respecially full of hot young large stars. However, most globular clusters are full of very old stars, which makes it seem unlikely to me that chain reaction supernovas destroy the entire cluster very often.