I had a similar situation today.I

My garbage disposal water was venting and backflowing into my laundry room sink. After removing the p-traps for both laundry and kitchen, and putting snake augers through them, I found out I had to go ON MY ROOF to put the auger through the pipe sticking out of it, and break up some compacted food.

The neighbor said this happens when you don't run enough water through the disposal. The houses and piping systems are old too, so nothing literally more than "egg shells" should even go down the drain. So much for the actual disposer.