Does the optical mouse use a USB dongle?
If you have USB ports on different sides of the computer try plugging the iPad in a different side than the mouse's dongle. If that works, I've got two possible (very speculative) answers.
One. The USB controller is older, faulty, whatever, and can't handle the data rates of both devices.
Two. The wireless signal for the optical mouse dongle is being interfered with by the iPad's cable. This could be because of a minor fault in the cable, bad USB port grounding, or the dongle could have very poor frequency rejection.
That's all I can think of for now.