I agree wake turbulence could be a big factor. I want to add though that the last recorded instruction to the pilot was to extend downwind and report the GLF5 in sight. After a few attempts the pilot says he has the GLF5 in sight and will follow. He turns in by himself and follows to close. As she did not initiate the turn the wake turb sep is on the pilot and no caution call is required (as far as I can tell). If he had said I have the GLF5 in sight but did not initiate the turn she would have said "Follow the GLF5, caution wake turb." Was it a good idea to give the caution anyway? Probably for CYA. Hopefully this doesn't come back to bite ATC. The amount of read back errors she caught and corrected was huge.