The following is all based on the assumption that your tenants are NOT overloading the breaker. If they use more than 15A, then you have to educate them or upgrade the wiring.
There is at least a 50% chance that the breaker has gone faulty. I have seen this happen several times. Some last 30 years, and others 2 years.
--> replace breaker.. 1 guy for 10 minutes plus $40 for the breaker..
Next possibility: one of the outlets has faulty or lose wiring. Modern ARCfault breakers will trip from this but they have been around for 10 years only (not sure?).. unlikely that your building has those.
Worst case: a rate chewed on the wiring somewhere in the wall and causes an occasional short, like when a heavy truck drives by. I currently have exactly this situation, but in a basement where the wire is exposed. If the damage is inside the wall.. good luck.. have to rewire the entire rooom.
Costs maybe 2 guys 6 hours?
By the way..
The rating of the wire is printed on the wire. You can see it by opening an outlet -or easier- at the breaker itself. It is unlikely that you have 20A wire and they put in a 15A breaker.
Reason.. thicker wire is harder to work with (does not bend easily..) and makes installation of those blue boxes for outlets more difficult. Once the installer has gone through the hassle of installing 20A wire, you bet they will put in a 20A breaker.
Unless someone replaced it years later...
Good luck.. you might get away with a $100 service call plus $40 for a new breaker!