Been there done that........ lots of times!!
I always get it when I use a brand of primer thats not plastic friendly, like auto primer. I have used several brands of primer and the one Im using right now seems to work best for me. Its Krylon Primer for wood,plastic, metal all around primer. It dries in about 12 minutes!
My guess is that since you used Tamiya brand primer that maybe it was some oil left on the body and reacted with the primer to form those cracks. Also laying on too thick of a coat of primer will either make the primer bleed down your car or form cracks where you might not have seen any if the coat was thin. But thats MHO and my
Take or leave it...
Scott