Not re Cassia, but re Brachychiton and perhaps of relevance ...
I have four B. populneus in a short row, ostensibly the same soil, situation, watering, etc.
They were all grown from seed collected at the same time from below street trees in Paphos, sown at the same time, planted at the same time.
One of them always produces its new flush of leaves later than the other three (which are pretty much in sync). I attribute this to chance minor variation in the species, and that pertains whether or not the seeds call came from the same particular street tree or not.
I see no reason why similar variation should not occur in Cassia or other trees.
But I can imagine environmantal factors could also produce similar results.
Mike