...doing wrong?

I switched over to tubs about 6 weeks ago. Since then, I have had 4 UTH's fail. When I had glass tanks, I never had this problem - my UTHs lasted for years. I don't know what's going wrong.

Each 52qt sterilite tub has a medium UTH on the bottom. I'm using bark as substrate. Each has a good-sized, heavy ceramic water bowl in it, which is perfect for maintaining the humidity. The UTHs are connected to the thermostat, which is set for 85; I have a back-up rheostat attached as well. I initially tried small UTHs, but those did not get the temp up and stay consistent.

Now, the only thing that seems to be different with this set-up opposed to the glass tanks, is that now my snakes are dumping their water bowls. They are the same bowls they've had for years, and they never dumped the bowls in the glass tanks. I mean that literally; I will find the bowls completely dumped over and water soaking everything and driving the humidity up considerably.

My husband thinks that when they dump the bowls, the water pools on the bottom of the tub, it heats up and burns through the tub, which then shorts out the UTH. Admittedly, there are a couple of small holes in the bottom of each tub and subsequent scorch marks on the UTHs.

So, what am I doing wrong? It is getting expensive to replace all of these UTHs.