Clearly, somebody TF had a strong enough motivation to do it. Not for ransom (unless stores are hiding part of story) just for the lolz or directed harm.

Could be competition, could be lonely angry incel hacker, could be state actor (ie russia would not be out of picture given their past actions and current war they wage against entire western culture, but they are far from the only ones).

Leave phpbb forum unguarded, it will get hacked quickly. What gain? None apart from hoping to mine some crypto or be part of botnet. Still, will happen reliably.