Are you sure they are pressed in? As far as I know the spiders are only held in the cup by the rubber and they cannot come out unless you take off the hub or cross arms? I used to pull them out and push them back in all the time when I would remove and install an engine. I would leave the cups in the gearbox to save the hassle of draining the gearbox oil
But still, only way for them to come out in a moving car that I could think of is a broken cup, or something else is broken/loose, or the drive shaft must have been at an extreme angle, perhaps the car is lowered?