Same thing happened to me with a Citi Card recently. Only thing I can think of is that I used it in an NYC taxi right before their systems got hacked. Thieves tried to make $2400 worth of charges on my card (which isn't unheard of when I'm trying to meet a spend bonus with business spending). First transaction went through, second didn't and tripped fraud alerts. Got the charge wiped off after filling out a form and mailing it back.

The funny thing had been that Citi called me twice on online purchases to verify if it was valid, and it made me upset since the first time the transactions wouldn't go through.

I check every 1-3 days on Mint and have PageOnce alert, so I could find fraud myself hopefully.