Eventually derailleurs wear out. There are a bunch of pinned connections that are exposed to the same grit that chains are. When these connections get worn, the derailleur gets sloppy. You can tell if you grab the parallelogram and twist, you can feel slop in one that is burned out, a new one will be tight.
You can check the friction in your housings by grabbing the cable and pulling it first from the RD end, and then from the shifter end. No friction = easy pulling.