Most septic failures are caused by human error or neglect. The primary culprits include: Inappropriate Disposal:

The most obvious and unpleasant sign—wastewater backing up into the lowest drains in the home. Lush Patches:

A septic tank is a type of underground container that holds wastewater from a household, allowing solid waste to settle and separate from liquid waste. The solid waste, or sludge, settles to the bottom of the tank, while the liquid waste, or effluent, flows out into the drainfield, where it's filtered and treated by the soil.

This is the homeowner’s nightmare scenario. If the clog has pushed solids out of the tank and into the drain field pipes, the soil around the pipes can become clogged with biomat—a layer of sludge that prevents absorption. In these cases, pumping the tank won't solve the problem. The drain field may need to be moved or replaced, a project that can cost upwards of $10,000 to $20,000 and require heavy excavation equipment to tear up the yard.

Do not drive or park on the drainfield, and keep trees or shrubs at least 10–20 feet away to prevent root damage. How to Care for Your Septic System | US EPA

Bubbling or gurgling sounds from toilets, especially when other water fixtures are in use.

Here is a deep dive into diagnosing, fixing, and—most importantly—preventing a clogged septic tank.