I can only guess dream. Typically during landing gear extension the landing gear is often dumped at the same time as some flap/slat which puts a load on the green system. The PTU jumps in if the pressure differential exceeds 500psi between the two systems, so I could only summarise that if the yellow is running at a slightly higher pressure, green drops a bit when the services are called for then you get a few moments of PTU. End of the world? I dunno, not an engineer, but I guess not. Like you I've never seen HYD PTU either, more things to look at!