In "Descent" why did the renegade Borg retrieve their dead the first time, but in the second attack, they didn't?

When the Enterprise surprised the borg during their attack on the MS 1 Colony, the borg did not want to risk going into battle against a top-of-the-line federation starship. So they made an effectively knee-jerk decision to send over and sacrifice two of their own simply in order to create enough time for their ship to make a quick getaway. The only reason the two drones were left behind was because by the end of the firefight, the borg ship was no longer around to pick them up.

That these borg were exhibiting such un-borg like behaviour was puzzling to the Enterprise crew, but that was what they had to think.

That was what they were intended to think. Right from the start. You see, EVERYTHING in the opening paragraph was fake - to take a couple of my quotes :

“Enterprise surprised the borg” - nonsense; the borg had been baiting the Enterprise precisely to bring on this confrontation. After all, Picard just a few moments earlier had said that the borg were only attacking when the Enterprise was the only ship around :)

“The borg didn’t want to risk a battle” - absurd; the borg ship was FAR more powerful. In Descent part 2, the Enterprise’s weapons couldn’t make a scratch. It had no reason to run away, let alone sacrifice some of its own crew to do so,

So what was really going on ? In a word, Lore.

Lore wanted Data by his side. In the opening firefight, the borg named each species in the landing party - and identified Data. Lore and his Borg now knew exactly where Data was, and everything that followed was PLANNED by Lore for the sole purpose of getting his borg onto the Enterprise, where they would be in a position to influence Data using the emotion chip.

Military deception, a plan within a plan - and Picard and co fell for it Hook, Line and Sinker !