I am of the opinion that the bubbles werwe blown by the starburst that led to the formation of the young galactic clusters in the bar and the inner ring surrounding the central black hole.
A collision & merger between 2 black holes (even a big one and a globular cluster intermediate) would result in a very low precession rate for the resulting merged black hole.
The current black hole is still precessing in all 3 axes (disco mode), as can be seen by the hot X ray emitting gas in its near black hole lobes (which are not aligned with the galactic disc); so the probability of a merger is vanishingly small.
An alignment of giant lobes with the galactic disc is however typical of a classical starburst of M82 type, as a result of the gas density in the galactic pole being less than that in the equator & bar (path of least resistance).
The cause of the starburst would be relatively simple: funelling of gas & dust inward along the bar as a result of tidal disruption of orbits due to interaction with the LMC & SMC. This interaction has already caused an approximately 2Kpc up & downwarp of the spiral arm bearing dust disc in our location leading to skewed out of plane stellar associations such as Gould's belt when star formation occurs in 2 warped ripples bouncing in & out of plane colliding @ some point in the warp bounce.
That same tidal disruption is more than capable of jogging inner ring gas & dust into the bar which then heads to the core from opposite directions after starting @ the ends of the bar. The starburst results when the infalling gas & dust collide in the middle of the bar.
The fact that our galactic black hole is small induicates that the current influx of gas, dust & young stars is NOT a usual occurrence, and that an external disturbance (tidal interaction) is the most likely cause.
Do not dismiss the ENTIRE story... A galactic collision (probably 2 of them) is due when the LMC & SMC loop back around and pass through the disc of our galaxy. The LMC & SMC are likely not to be independent irregular galaxies for very long in the future.
Have a nice day: Ag