Worn out connector? Pick up a set of new ones from Autozone for $30. Get the Bosch with the lifetime warranty. If it happens again, they'll replace them for free.
Might help to know if it's popping off the plug or the coil, but depending on age, it could be something else needing replaced as well.
Also I have found that if you do not relieve the air pressure inside the coil terminal boot when you install the plug wires, the heat is enough to expand the air and push off the boot. I just pinch the boot and you will hear a hiss when the air comes out. I've had this happen once on my car and twice on my dads old ram van.