The answer to 'why is my sewer line low water pressure' is that there are several common causes, including a clogged or partially blocked sewer line, tree root intrusion, collapsed or broken pipes, or a problem with the main sewer line connection. Low water pressure in the sewer line can lead to slow draining sinks, bathtubs, and toilets, and may even cause sewage backups.
To troubleshoot the issue, you'll want to start by checking for any visible clogs or blockages in the sewer line. You can also try using a plumber's snake or auger to clear the line. If that doesn't resolve the problem, the issue may be with the main sewer line connection or a more serious pipe problem that requires professional plumbing services.