Very interesting, Splugged. Can you see any particular pattern to the search traffic during those "rush" periods? Particularly, does the keyword proportion change? If so, that would seem to indicate some kind of SERP change.
From observations shared here recently, it sounds as though some updates phase in gradually. That "phasing in" involves shifts that include very weak SERPs on some keywords that used to have a stronger set of results... and then the SERPs eventually settle down again on a stronger set. Somewhere in the middle of the process, it seems to mess with things for a period, as well as making for surges in search traffic.
Have I represented the observations accurately?