In this case you have a national roaming, yes?
“… This can be due to (as NK said) some problem with the BTS (but i don’t really see how…) …”.
If in dense urban area we have one BTS down than nothing happen – all MSs will reselect the BTS following the reselection criterea.
If we have some BTSs down ( or one in rural area ), so we have a MSs without coverage.
In the moment of BTSs up there will be a large number of network registration procedures with all attended events…