Dishwasher - water is out the air pipe

[George P.]

The tenant has informed me that the dishwasher (4-5 y.o. Kenmoore) has started pushing the water up the air pipe.
My initial reaction is the diahwasher's sewer pipe is clogged. But what else could it be?

There is a disposal that the sewer pipe is connected to (I think).

I am going there over the weekend to take a look and possibly fix.
Thanks in advance!