As NK said, it certainly has to do with a massive amount of location updates (you can see perhaps that the SDCCH were allocated for LU ?)
The main possibility is that at 1am one of your competitor operator had several BTS down, de to BSC outage, or Abis outage probably. At that point, all the subscribers of this operator will search for another network and probably select your cells. Once the BTS were up again, subscribers would gradually reselect automatically their home BTS (timer defined in MS).
Another possibility is that your own subscribers performed a location update all at the same time. This can be due to (as NK said) some problem with the BTS (but i don’t really see how…) or at NSS level : the MSC asked many MS to perform a location update at the same time. Were there any MSC action last night ?