Ok a large amount of roots just does not appear over night. It takes years to grow from a small amout and snaking the drain like maybe once a year. Then as time goes it becomes a bigger mass and your snaking it twice a year.
I have never seen or heard anyone try and claim former owner responsibility. Most drain cleaners will be able to tell you if the line has been opened in the past. The clean out will be marked or a newer style.
I'm betting there is a rather large hole in your pipe.
Have you thought of or heard of pipe relining or pipe bursting. It could save you a few bucks.