To do the balljoints, I would guess they had to take out the axleshafts, and if they drug them over the seal, that would damage the seal. I would question their competence simply for not replacing the seal. Any competent mechanic would STRONGLY encourage you to replace those seals as they are extremely difficult to get to, and fairly cheap and easy to replace once you are in there for balljoints.
That would be a fantastic coincidence to have a seal develop a leak after they worked around it, but one thing to be aware of, is that if there were a leak before, you might not have noticed it, but then did afterwards. Again, any competent tech would have brought it to your attention before finishing the job as any preexisting leak would have been obvious.