Alls well that ends well I guess.
As for the cause for nanners its anyones guess.
Its just my personal opinion only about herming but here is my thoughts.
I think theres just underlying mechanisms in canna that get switched on occasionally.
When they appear during stress of stretch I think its due to the changeover of reproduction hormones coming online. Like puberty in humans. Plants have been evolving and surviving at least 100s of thousands of years, so I think they prepare for sexual maturity by pushing out an odd pollen sac.
Then when the annual change comes on in late flower, faced with demise, it is the next best time to have a last ditch effort to produce a seed.
From their viewpoint, the propagation of the species, succession of genetic transfer is the goal. I think they make latent their self reproductive ability, in this case a nanner, to maximise the chances of success. Think of the catastrophic conditions they must prepare for to be successful. Drought, fire, flood, snow, low dlh, predation, pests, disease.
And viewed from that standpoint it makes sense. Light stress, droughting, heavy defoliation and pruning late, all may have an effect on them to decide to turn that on, or more accurately make latent. Because it plays its part as a reproduction asset, both as an individual, and as a part of the collective.
Foremost it wants to produce a crossed offspring, to increase its diversity. But failing that it will self to ensure another chance to be viable, or drift female derived male pollen to its immediate neighbouring kin.
We want femmed sterile grows selected for superior harvests, they play the survival card as the two objectives are wholly differing, at odds with one another as objectives, and nature is on their side.
Some gentic lines are much more susceptible.