Is there a reason unsolicited offers for domains seem to come in waves or has it just been coincidence for us? We go long period of no unsolicited offers and then half a dozen in a month. There's no SERP, publicity, or traffic changes for these domains that justify the cyclical nature of it.
I think it's more a function of long lists of domains, the entry of new players, an attempt to appear unspammy by turning solicitation campaigns on and off and on, some amount of "checking back in just in case", a certain amount of workers being assigned busywork during any downtime, add to the mix a bit of the relentless work of mindless domaining machines (think "domain tasting, Ver 2.0) . . and that about it explains it . . :-/
Like any form fo spam it's a bit annoying but it's easy enough to spot and filter.