I'm opening new paint tubes for watercolors, and about 50% of the time, the paint starts gushing out slowly. I then hear a few pops. This would be okay if it only happens once when I open the bottle. I opened the black paint tube yesterday, no gushing whatsoever. Today I open it, and the entire inside of the lid and screw threads on the tube are completely submerged in black paint, and there's a worm of paint slowly making its way out.

Why does this happen, and is there any way to prevent or stop it?