1. To come apart into pieces. We need to get this wall repaired—the drywall is just crumbling up.
2. To break something into smaller pieces. In this usage, a noun or pronoun can be used between "crumble" and "up." I crumble up some cookies and add them to the icing—that's my secret recipe.
crumble something up (into something)
to crunch up or break up something into pieces. Now, crumble the dried bread up into crumbs. Crumble up the bread into crumbs.
to break up into little pieces. The cake, which was very dry, crumbled up when I tried to cut it. The paper of the old book crumbled up when I turned the pages.