Because the bridges to the other planes have been removed, souls cannot pass from the material plane to the aether and nether without help.
The Sending is a formal ritual designed to help guide souls to their rightful place, and must be completed by a powerful cleric. However, when one is not available, or the death of something is inconsequential, an informal ritual can be completed by anyone who knows the method. The soul is not guaranteed to work, though, and therefore has a chance of leaving behind the soul.
Souls unable to leave the material plane have a chance of becoming a spirit.