There are a lot of feral cats in our neighborhood. Here lately I have noticed that some of the cats are walking around as if they are drunk and progress to the point that they can not use their hind legs to walk. Some try to walk and fall down. Why?

It sounds like someone poisoned them.  Call animal control or a cat welfare organization.  At a minimum, someone should trap them and spay or neuter them to control the population.  They can get a rabies shot at the same time. 

Feral cats have short, difficult lives.  Feral kittens can be easily socialized if adopted.

Poisoning is about the cruelest thing someone can do.