Six millenia ago, Proto-Indo-European speakers invented the word grumo. This was a weird word; according to its reconstruction, it meant "scrape together", "scratch", or "claw" in addition to "lumber" and "junk". This eventually caved into the Proto-Germanic word krumo, which meant "fragment", since the "junk" you "scrape together" are "fragments" of "lumber", I suppose? That would be ideal, but it probably only developed from one of the aforementioned definitions. None of my sources know what happened next: the word was marked as "obscure" until it mysteriously resurfaced in Old English as cruma, still meaning "fragment". Here the word split into two. One one hand, the word morphed to crymel, meaning "to become fragments", another instance of anthimeria. From there it changed several times, incorporating the Old English term crymlan to the Middle English term crymblem to the more modern word crymble, to crumble, which is what it is today. Makes sense. Meanwhile, as crumble was developing, so was crumb, with one major difference. It went from Old English cruma to Middle English crumme, with no addition of a b. Later, as Modern English developed, it adopted that b under the folk etymological influence of crymble to become today's word crumb. It's fascinating how these words influence themselves over time to become what they are today.
Adam Aleksic, a rising sophomore studying linguistics and government at Harvard University, has been described as the internet's sixth most famous etymologist. He also has disturbing interests in politics, vexillology, geography, board games, conlanging, and law.