I think what OP is trying to point out is that, upon multiple banks failing--be it a few to a dozen--there is evidence that the number of failures has a cascading effect. It seems to me (=opinion, not certified) that there is a certain threshold whereupon the number of bank failures is detrimental enough to the economy (or other banks) that there are more "waves"--as OP put it--of bank failures in a short amount of time. I think they (OP) used the long time span to signify that these "waves" (or clusters) of failures are repeat occurrences.