In Uriagereka's 1999 article, Multiple Spell-out, the term cascade is used in several places. I just conjecture its meaning but don't get exact one. What does a cascade mean in syntax.
Actually, it is a derivational cascade. I think it's Juan Uriagereka's terminology. Uriagereka 2005 writes that command units (or CU's, objects formed by the continuous application of Merge to the same object) are spelled-out at the end of each derivational cascade. The more common term in the Minimalist Program is a phase.