Sump pump (blockages, float get stuck, not powerful enough to handle water volume, not throwing water far enough away from house, check valve loose)...one you have eliminated all sump pump nonsense, go 2.

Initially thought it was a vent blockage, but we had a camera run through the lines, and there is a 3 foot section of the main sewer line (cast iron) with trench rot.

Ultimately the plumber discovered it was a building pipe blockage (we are in 3-unit building, we are on ground floor so the pipe affects us first).

Cast Iron sewage line seems to be causing repeated blockages and the basement tenant can't stay while things are being repaired.

Others do it on site and you have to be careful of the way they dispose of their waste, especially liquid toxic waste.Some time back, I read of a lawsuit here in NY where the landlord and his dry cleaning tenant was sued over flushing hazardous materials done the drain, and then a sewer blockage caused contamination in the neighborhood when there was wastewater backup.

Your time is too valuable to spend it chasing down tenants for payments.Make it clear who (tenant) is responsible for plumbing blockages, which is the most common service call a landlord gets.

BABY WIPES (EVEN "FLUSHABLE WIPES"), FEMININE HYGIENE PRODUCTS, HAIR, KITCHEN GREASE, AND A WIDE VARIETY OF OTHER THINGS WILL CAUSE DRAIN BLOCKAGES. 

How do you know which resident has caused the blockage?

We haven't scoped sewer lines yet because our local plumbers balk unless there's a blockage.

My agent tells me they had a plumber come out and fix this issue but there may be a blockage further up the sewer pipe leading into the street.