Why does this formatting problem occur with the forum module? It doesn't happen every time the module is loaded, but it happens many times. Notice how some of the forums only expand to ~1/2

First things first, what DNN version and what module version?

Please also try the same instance of a module on a different page w/ no other modules. Next, try turning off the footer via module settings. Does it still happen?

In the past I have only seen this as a problem in one of two situations:

DNN forum version and DNN Core version compatibility issues (membership API changes in 3.3.x/4.3.x)

Someone puts in subject content that has <td> values but those are html encoded at display time. (All subjects are html encoded at display time to avoid people using scripts)