In my own mind, I just kinda said that they are constantly assimilating new species, so there must be some influence from the new information they gain, and this may be one of the ways that that new info has been processed into the collective and dealt with.
This is a very logical and reasonable explanation. Was it ever established that the Borg assimilated any of the major races prior to the TNG crew's first encounter with them? The reason I ask is this; perhaps, when they first assimilated species like the Klingons or Romulans some of their more aggressive character traits were imparted to the Borg. This, in turn, modified their behavior and they evolved to what we saw later.