Root intrusion at a joint or a hairline crack is the most common cause, particularly on an older run near established trees. A pipe section can also settle unevenly across the decades, sagging into a dip that traps solids instead of letting them pass through, or a joint can pull apart slightly as the surrounding soil shifts. All three produce the same surface symptom, several fixtures draining slowly or backing up at once, which is why we scope the line with a camera before any repair gets planned, not after.
Digging on a guess wastes yard and money in roughly equal amounts. A camera locates the actual defect and its depth, and it also shows whether the run is a candidate for a trenchless fix or genuinely needs to be opened up, a decision worth making with footage in hand rather than an assumption.