Here's a guess: the pretzels were probably really heavily coated and shiny so parts of their surfaces repelled the butter instead of absorbing it to hold the crust together. Does that ring true? You'd probably want to use a pretzel with a flat rather than shiny surface for the crust.

I agree with drbabs on the coating. Another possibility is that the pretzels weren't crushed fine enough.

Yes, that was my other thought, TiggyBee.

It was shiny, and not all the pieces were fully crushed, so I'll try crushing them finer next time. Thanks!

good idea though! did you use salted pretzels and turn it into a sweet/salty combo?

Yup, that's exactly what I did. Put a chocolate layer on the bottom of the crust and then filled with peanut butter cream. The problem with the crumbly crust was that the chocolate was really hard to spread out and ended up lumpy, but still very tasty.