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Why are these relevant or special for the GC in a "gotcha" manner?

Because a static reference causes objects referenced by it to never be GC'd. As a simple hack, someone might add a singleton implemented as a static field of a class. That reference could be/have a structure/collection that accumulates and never gets freed.

You can find these things using the Java Memory Analyzer and the OQL it supports. Sometimes easy sometimes tricky.

Presumably if you're creating a large map in memory it's being used, no? Or did you have something static that was really only used during startup?

Classes can also be GCed in Java, but obviously only if they're no longer referenced.