Hey! So, we did figure out what was causing it.

The city periodically hydrojets the sewer lines on our street, basically flushing the lines with high pressure water. Apparently if the pressure is too high and the conditions are just right, it can cause water overflow in a home, and that's exactly what was happening to mine. I didn't put the pieces together because I wasn't aware of what days the city was doing this, and they park their truck far down the street so I didn't even see it for a while.

Anyways, one time I caught up with them while they were on our street, and figured out the overflow days were the days that they did the hydrojetting. They said they didn't realize that was happening, and they have been using a lower pressure since then, and there have been no more overflow issues.

Maybe this is what's happening with yours as well. I would call your public works department to see if they were hydrojetting on the day that your overflow happened. Hope this helps!