The theory goes like this: On stardate 42761.3, the U.S.S. Enterprise was once more visited by the mysterious, omnipotent being "Q", who stated they [humans] were not ready for the bigger dangers of the galaxy. When Captain Picard assured him they were, he flung the Enterprise 7,000 lightyears away into a region of space occupied by the Borg, a group of aliens united by a hive mind who seek out technological perfection, which is why they had no initial interest in the biological component of humans or other aliens they encountered aboard the Enterprise. After obtaining a sample of the Enterprise, Captain Picard orders the Enterprise to flee, only to be overtaken by the Borg. It was only the timely arrival of Q who enabled the Enterprise to escape, no doubt much to the puzzlement of the Borg.
When the Borg first stepped aboard the Enterprise, they began obtaining information from the computers. Even if material was classified, enough decryption would undoubtedly uncover it eventually. Between the strange energy readings and what could possibly be hints of a strange being known only as "Q", the Borg grew curious. A being, a purely biological being, able to command power at his fingertips to create, change, manipulate and destroy matter and energy? This was something new, and hinted that perfection perhaps included a biological component. If such is the case, then it would become imperative to obtain as much biological information as possible, and that would require a new tactic: assimilation.
And so the Borg began to study the biological component. Species that already knew the Borg were dangerous because of their obsession for technology now feared being assimilated into the collective and made to work against their own kind. Each new drone was another processor, another test bed, something the Borg could use and study. Each mind was another processor, capable of working out solutions and trying to solve the riddle of how to obtain perfection from a being that could do such incredible things. The answer was obvious: Humans, specifically the individual Jean Luc Picard. Something about this individual was what brought a creature like Q to the fore, and so a new experiment was launched: The Queen. An individual, who could process information in ways that no drone could, in order to study the vexing problems from a unique perspective and possibly deal with Q similar to the way Picard had done.
And with these preparations in place, the Borg launched an attack on stardate 43989.1, sending a single Cube in order to threaten Earth and draw out Picard and/or Q for study. The cube lucked out in that Picard was obtained easily from the U.S.S. Enterprise, with the goal now to draw out Q. With his apparent favorite, Picard (Now designated Locutus of Borg), Q would be dealing with the Collective instead. Only it was the timely intervention of the Enterprise that caused the plan to fail, and so fall back plans had to go into effect. This included further study, advancement, adaptation, and allocation of resources to come up with a plan that would beat the Federation. Clearly, they needed something new, and so the plan fell into effect on stardate 50893.5 with the second attack on Earth.
The plan had the following objectives:
- Destroy Earth's defenses and begin assimilating Earth to draw Q out of hiding.
- In the event this fails, travel back in time to remove the ability for humans to ally with Vulcans.
- Calling on reinforcements from the Delta Quadrant, the Borg will use 24th century technology to assimilate the galaxy.
- With a head start of 4 centuries, they can easily conquer all and hopefully draw out Q.
While it's entirely possible this was only a joke, Q did utter the line to Q (junior) that he shouldn't provoke the Borg. It's possible, however slim, that the Borg could learn something on accident from one of them, and to leave them alone just to be safe. It also no doubt helps that Q is omnipotent and knows the Borg won't last forever. Still... if the Borg are truly out to get Q, then it would be smart to stay away from them.