That's my basic problem. Any ideas why this is happening?

"basic problem" is that the drain line after the tub is at least partially clogged. A washer pumps out a lot of water in a short time and the drain can't remove it fast enough. Tub level is below washer pipe and water backflows. Have someone auger out the line from a cleanout.

If the tub is the lowest fixture in the home then this suggests that the MAIN DRAIN line may be partially plugged. To check this, get water to back up only slightly into the bathtub and then remove the main drain CLEANOUT and see if there is standing water. If there is standing water in the main drain it will need to be snaked to clear the drain. If the problem persists then a specialized video camera may be needed to scope the issue out further.

Mark