Pressure Cooker "Safety Fuse Plate" Bubbles During Use
Beans and legumes produce a lot of scum/foam (it's a mostly denatured protein mat) while cooking.  For a pressure cooker, this can clog your vents (a bad thing).  So you want to ensure that not too much is produced.  Your user manual should have a section describing the cooking of beans and/or legumes with appropriate instructions.

An example is on page 23 of this manual.  To summarize:


Don't fill cooker more than 1/2
Adding a tbsp of oil per cup of legume can reduce frothing [I haven't tried this]
Bring the legumes to a boil and skim the initial scum before closing the cooker