You can really see the behavioral differences too, in their posture and attitudes, like how Timo
described it, going from robomen to zombies. They're really aggressive in the film.
That there I would attribute to needing to be more aggressive with Humans then with past aliens they encountered.
Back when Q tossed Picard and crew to meet the first Borg, the teleported to the ship, did their thing, didn't need to get physical all that much, because they felt they had everything under control and would be able to take them all out with little effort.
Then later on Picard/Locutus, Riker and crew destroyed a cube and screwed up all their plans.... then they screwed them up even more with sending a borg with individuality back to the collective..... then Lore screwed them up even more by getting involved and making them more aggressive..... that probably didn't give the entire collective an incentive of being more aggressive, but the encounters with Janeway in the Delta Q. how much trouble she brought to the table, how many times she tricked them, etc. etc...... I'd say that eventually the Borg lost patience with us and started to get more aggressive with taking out individuals and assimilating them, since these individuals seemed to be pretty crafty and dangerous.
Just a theory.
That also fits with the recent novels. In the books it got to the point that the Borg were so pissed(so to speak) about getting blocked at every assimilation attempt, that they ended up deciding to forget assimilating and decided to just attempt to eradicate humanity instead. They killed first and assimilated only if it was an advantage.