Its not randomly! If you punch in a number then a bill button, itll multiply the number by the bill amount. For instance, if you punch 20.00 and then hit the 20$ button, itll do 2000 * 20 and assume the person is paying with 40000$ cash. This is a shortcut way for if the person gives you five 5$ bills, you can punch 5 in the register (shows up as 0.05), then hit the 5$ button and it does the math.

Granted its super useless and I never had to use this and it caused more issues than anything else, its not random and knowing how it happens at least helps not doing it too much 🤷🏻‍♂️

Im pretty sure it happens when you double tap the cash options