Adrian Goldsworthy has a wonderful and quite accessible/entertaining (imo) biography on Julius Caesar, and in it, he discusses Labienus and Caesar's relationship at some length.

As I recall (it's been ~2 years since I read it, and I don't have the book with me), Goldsworthy suggests that Labienus may have had a falling out with Caesar--seeing himself as not getting his fair share of the credit in Caesar's Gallic Wars, not getting promoted appropriately fast for his liking and seeing greener pastures on the other side. He's also quick to note that, there aren't a lot of detailed or non-biased sources on the break.

More likely than not it was a calculated political move: at the beginning of the tensions Caesar was much more of an underdog than he's often given credit and likely had more to gain in prestige and political clout and office (Caesar had recently made it clear he was going to promote someone else to what Labienus, due to his age and service, likely considered his consulship) by winning with the Pompey-aligned forces.

We don't know Caesar's reaction to his death or switch to the republicans camp. However, I think it is safe to pull from the general political climate that Goldsworthy presents in his book for context.

Goldsworthy does a wonderful job of discussing the fluid, extremely personal, and purely self-interested politics of Late Republican Rome. Labienus wasn't the only person who had previously been close with Caesar that fought against him, he had a few relations that did the same, and often (though not always) made a very public show of forgiving them and others when apprehended then giving them clemency for their actions.

Hope this clears it up a bit, and would highly recommend Goldsworthy's Caesar, Life of a Colossus as well as his other books if you're into this type of detail (he's much more comprehensive than I have been!) and political/military interplay in your history books, as I am. :)