Transmission fluid leaks can be caused by a variety of factors, including:

1. Loose bolts or holes: Over time, loose bolts or holes may occur, causing the transmission fluid in your parked car to leak. Sometimes impact can also damage the pan.

2. Damaged drain plug: If too much force is used when changing the transmission fluid, it can damage the drain plug. The repair professional must tighten the drain plug according to factory specifications. Too loose or too tight can cause transmission fluid to leak.

3. Cracked or broken fluid lines: If the transmission fluid leak happens toward the front of the vehicle when its parked, it could be an issue with the transmission line. Most vehicles have fluid lines that run to the transmission cooler in front of the car. Most of these lines are designed to last many years as theyre made of steel or aluminum. However, they may crack or break at any time and eventually cause a leak.

4. Damaged seals: An automatic transmission contains seals designed to keep the fluid in place and prevent the fluid from leaking. However, these seals deteriorate over time and develop cracks that lead to leaks. The most commonly affected seals often lead to leaks at the input and output shaft seals.

5. Clogged transmission lines: Like engine oil, transmission fluid can be subject to dirt and debris buildup over time. If ignored, the transmission lines can become clogged, and the excess pressure can cause the pipes to burst or crack, causing a leak.

6. Overheating: If the transmission fluid only leaks when the vehicle is parked, its a sign of overheating. The transmission includes several seals to keep the fluid from leaking, and many of them are affected by high temperatures.

7. Punctured pan: Road debris can damage the transmission pan, causing transmission fluid to leak out of it. The transmission pan also has a drain plug to drain the old transmission fluid out of the pan when it’s time to replace it. Fluid can leak out of an old plug, too.

8. Worn-out seals: Your vehicle’s transmission has a series of seals that maintain the hydraulic pressure the transmission needs to function. These seals wear down over time and fluid will leak from the driveshaft, input shaft, output shaft, or from other transmission parts depending on where the worn seal is located.

Its important to address transmission fluid leaks promptly to avoid further damage to the transmission. If you notice a leak, its best to take your vehicle to a professional mechanic to diagnose and repair the issue.