The importance of the "max fill" line in an electric pressure cooker depends on what you're cooking. If you're cooking legumes, you should respect it, because they can foam and make a huge mess if you try to cook too much at once. But I've never had a problem cooking steamed vegetables past the line.
In my experience, going past the max fill line is mostly a problem when using the “rapid pressure release” on a recipe with lots of liquid. Pressing the release button can emit a a superheated geyser of soup with enough force to hit the ceiling.
When the timer goes off and the cooker switches off, the contents are still under pressure. That pressure shifts the liquid's boiling point, which is why pressure cookers cook hotter. If you vent the pressure vessel, the liquid inside will rapidly boil as it dumps that excess thermal energy to steam and your kitchen. It'll carry bits of liquid and cooked food with it.
Another thing you know but may not have connected: this is exactly the same process that can make water "explode" if you reheat it in a microwave.
I think the microwaving water issue is slightly different. It does indeed become superheated, but not of course from pressure; if the vessel lacks nucleation sites (imperfections) for bubbles to form, the water can heat quite a bit higher than 100C as it’s not losing energy to the steam phase change. Eventually when a bubble does spontaneously form (or the vessel is jolted), a huge amount of water is instantly converted to steam.
Pretty much everything inside the pressure cooker is a nucleation site, so if you could make the lid just disappear, the superheated contents would explode.
Water is only converted to steam while there's energy for it, so only a fraction of the water actually boils*. The risk is that it's a rapid process that propels a lot 100C water outside of the vessel.
*) I haven't done the full calculation, but I'm pretty sure the upper bound is something like 10%, with 2-4% being the best most normal people could accomplish in a microwave.
On a somewhat related note, I actually had this happen recently on the stove. I've done it a few times over the years in the microwave, and I didn't even think it was really possible to have happen on the stove.
Plain stainless steel pot, small, good condition but still a couple years old. You'd think plenty of nucleation sites, right? Filled it with 195F water from the insta-hot tap, and put it on my induction stove and turned it on high to push it up to a roiling boil. But after 30 seconds -- nothing. Not a single bubble in the pan, which is weird all by itself. And at this point the range is pumping a fairly ridiculous amount of energy into what is already hot water.
So I thought "it can't really be superheating, right? on the stove??" and then grabbed the pot and swished it really good while stepping back. Sure enough, it instantly erupted into a furious boil.
I'd guess I'm unlikely to ever run into that situation again. The stars had to align pretty well, I think.
the clausius-clapeyron relation is part of bumping (the water at the bottom of the vessel is under higher pressure than water at the top) but the bigger reason for bumping is, I think, superheating
What's nice is when this happens by itself while it's cooking and it spackles your counters, cabinets, and maybe even ceiling with salsa and food debris while you're away.
