Banking shenanigans. I've just learned that charges are not actually necessarily deducted from an account in the order they are placed, but holds are placed until those charges are actually deducted. Now, my credit union has a thing where they'll pay charges even if you don't have the available money, but they charge a $25 fee for every such transaction. However, what triggers the fee is the worst possible metric for members - it's every charge actually deducted, when the charge exceeds your current balance minus your holds.
As an example, what happened to me. I had a balance of about $130 yesterday. I drove to my hometown to visit friends, and do a bit of shopping I can only do there. So I spent about $50 on winemaking supplies to last a couple years, $10 on miscellaneous purchases throughout the day (about 3 or 4 totaling the $10 when summed), $40 on a nice dinner, and $20 on gas. By my reckoning, I had $10 left, and indeed that would've been fine if my cell phone plan hadn't made a charge of $20 at about 2 AM because I forgot to disable the auto-charge (this is entirely my fault). So the winemaking purchase and the gas had already been charged, and now there's a new hold; when the dinner charge is actually deducted, it triggers the $25 fee, giving me a negative balance (even though I wouldn't have had one without the fee), and I've now got 3 to 4 additional charges that, if actually deducted today, will run me $25 apiece. If deducted tomorrow, I'm fine, but it's a work day today so I don't have time to carry out any of the possible shenanigans to get my balance positive (all I've got left is a giant pile of recyclables, and the possibility of donating plasma, neither of which I can do in the hour I have left). If deducted today, I'm an extra $100 dollars in the hole and I can't really afford that.
All I can do is wait and hope.