When I used to eat pizza more, I used to run into that once in a while.... very slippery toppings. Not so much when you were just transporting the pizza, but when you were taking a bite or trying to separate a slice. I blamed it on the cheese. Some places use a mixture of mozzarella and provolone and the cheese seems to melt back together after cutting, or it doesn't entirely break away after taking a bite, as a pizza with straight up mozzarella. So when the slice pulls away from your mouth or the rest of the pie, the toppings are dragged off if you're not careful.
And then again, some places put some ingredients under
the cheese, especially if you are getting several meat toppings. This of course could put a layer of grease, errr, meat juices, between the dough and the cheese. That could add to the problem.
Or maybe there's just a lot of sauce on it