Why does my bank keep flagging things as potential fraud?

I’m calling my bank tomorrow morning but I am so confused.

I am not asking for specifics, just maybe some general reasons things get flagged. I’m sure the specifics depend on the bank.

I tried to buy a $60 game online but it wouldn’t go through at all. Yet a $70+ purchase online on something else was fine.

I’m sure it partially depends on the bank but are there some universal reasons for why things get flagged? I wonder if it has to do with the seller originating out of state but it’s Nintendo not a third party thing. And it’s not Nintendo of Japan it’s Nintendo of America so it’s still in the country.

I also get flagged if the seller is out of the country but that was for something entirely different.

They put my card on hold for the game, I verified it was me but it never went through and I still couldn’t buy it.