Inexplicably specific fraudulent charge?

This has been puzzling.

On Saturday, my sister wrote my mother a check for $1,100, which I then deposited into my mother's checking account using a mobile banking app; it's the bank's own app, the same we've always used for this sort of thing. Essentially a money transfer between accounts. The check came from my sister's own checkbook, which is in regular use.

On Tuesday, my mother goes to make a purchase and is rejected. Upon further investigation, she finds that her account has been locked because of a suspicious charge: someone in China had attempted to make an $1,100 charge for something in Japan. We are from the U.S.

My mother's account had well over $1,100 in it.

How would someone have the means and intent to charge exactly and only the deposit amount to her account? If they'd had access to the account - if, for example, her card had been copied - shouldn't they have drained it? If some part of the check deposit process was used...how is that possible? It was my sister's check. It should have nothing of use for accessing my mother's account, right?