Anal leakage is a medical condition where the anus has lost elasticity and can no longer perform reliably. This condition affects about 2% of all adults, usually older people.
Anal leakage usually stems from damage to the anus. This damage can be done by anything that strains the anus. This can happen in various ways from child birth to anal dilation not done with proper care. Some websites also list anal sex as one of the possible reasons, however there is no conclusive proof and a lot more web pages state that anal sex is perfectly harmless if done correctly (don’t forget the lubrication if it hurts!). The problem can become worse if you drink, take some medications or eat certain foods.
There are no real treatment for anal leakage. Silicone injections can help reduce the problem some but do not solve the issue 100% of the time. Modifying your diet is also an option if the problem is not very severe.