Some other likely reasons:
1) Chances also are that they were doing an update/upgrade. If this is the case, chances are that they didn't take into consideration that they should have done it when they have the least traffic (like in the wee hours of the morning, on certain days).
2) Could be a server hiccup, maybe caused by power failure or switching to a gen.
3) Server crash: This might have required a reboot.
4) Could be the server is shocked, courtesy of Sanusi unleashing EFCC on bank execs

Be it as it may, if it happened during normal business hours and it rarely happens, I will give them a pass. Otherwise, it is inexcusable - a bank website is a mission-critical application and so they should have built in fail-safe policies and redundancies to deal with all possible issues.