#1 Clogged Baffle or Pipe
If a clog occurs in one of the pipes leading into or out of your septic tank, the whole system will stop moving and the wastewater will have nowhere to go. This is a common cause for water to back up when you try to use your drains.

The septic baffles, which are located at the inlet and outlet to the septic tank, can also become clogged. This is particularly likely if the baffle or the pipe is installed incorrectly. If the baffles are made of concrete, they can eventually crumble in response to the hydrogen sulfide gas in the tank, blocking the entrance or exit.

#2 Crushed Sewer Line
The main sewer line drains your home's wastewater away from your home plumbing system and dumps it into the city sewer line or your septic tank. Often, these lines are made of clay, which can be crushed or snapped by heavy pressure from above. Once the line is crushed, the route to the septic tank is blocked, similar to a clog, which can cause septic backups.

Driving vehicles over this line can easily crush it. Typically, you should avoid taking anything heavier than a push lawnmower over any part of your septic system. But these lines can also be crushed from underground if a tree is close enough to send woody roots into the pipe. A root intrusion won't likely destroy the pipe right away, but it can do so over time if not caught in time.

#3 Septic Drainfield Failure
The septic tank, the best-known part of the system, is actually not the part that does the most water treatment. Instead, the tank separates solids from liquids and collects the solids, sending the wastewater out to the drain field for treatment. So if the drain field fails, no water is going to get processed and treated at all, which can result in serious backups.

Fortunately for the conscientious homeowner, the most common causes of premature drain field failure are perennial neglect and misuse. So if you've been keeping up with maintenance and not driving over your septic field, drain field failure is unlikely to happen. The bad news is that a drain field's lifespan is around twenty years, so if yours is older, it may be on the way out.

#4 Overwhelmed Septic
Even if your septic leach field is functioning normally, it's still possible to overwhelm the system with too much water. This is especially common during wet, rainy weather because the septic system disposes of wastewater by allowing it to soak into the ground.

The water treatment process has to slow down if the ground is already saturated. It can sometimes slow almost to a stop, meaning that any water you send down the pipes will just accumulate in the septic tank until it's full and then start backing up the pipes.

You can help to avoid this problem if you cut down on any water-heavy activities when your yard is swampy. You should also avoid doing too many water-intensive activities on the same day (such as taking a bath, doing several loads of laundry, and running the dishwasher) even when the ground isn't so wet. Try to space these activities out more.