why do the edges of hostas turn yellow?

Answer 1

Depending on what variety hasta you speak of, some do naturally turn yellow during the season. if they are yellow from neglect, it's MOST likely, from too little water.

They are hearty plants, you can almost never kill them
The sun wont hurt them and neither will watering a lot.

BUT not enough water can make them go into hibernation pre-maturely.

Answer 2

Could the variety of hosta you have or it could be too much sun or not enough water.
Hard to say without knowing the exact conditions they are in. Here is an article that may help.
http://nga-gardenshop.stores.yahoo.net/hosta-l-g.html