Addicted gamblers have different mindset, they will do everything in their power to have the money to continue with their gambling, no matter what the cost is, including stealing, or borrowing big money from others with no intention to return it even if they win.
So this is just a classic example, the teller see those accounts that have huge money and so she put up a plan that she is going to steal from this unsuspecting victim. And her plot works and she was able to withdraw money, until such time that she was caught. Just imagine if the banks didn't notice it or the victims themselves, she could have siphoned all the money from the old and fragile victims.
They will think how they will have money to continue playing gambling even if they must steal money from other people. They can do bad thing easily because they really want to return to casino and continue their gambling activity. That will be dangerous and risk for them because they can get in the jail when people catch them and gives a problem to themselves. If they borrow money from others, they will not think about return the money and will run away without paying the money.
She doing a wrong thing to fills her gambling addiction and thanks to CCTV on the bank that caught her doing that thing. She know about the system in the bank so she thinks that is her opportunity to have the money by stealing those accounts. Many things can be done by addicted people to gambling without we can imagine because they can find that way to have the money. We guess that she has a heavy gambling addiction that needs to cure.