I have lived in Ohio, Missouri, Colorado, and Virginia. I have never heard of the verb “tump” in my life, until now. If it was included in The American Language by H. L. Mencken, I must have missed it. All I knew is that a “tumpline” was a strap across the forehead, used by Indian women to help support loads carried on their backs. From an Algonquian word, mattump. But that carries pretty much the opposite meaning: holding something up instead of capsizing it.
Which part of America? I’ve lived in the east and west and visited the northeast quite a bit, and have never heard “tump over”.