Too much mulch!
Adding mulch to your beds every spring is a great idea for a number of reasons--it helps the soil retain moisture, regulate temperature, suppresses weed growth, and makes your beds look great!
One important thing to remember about spreading mulch though: keep it off the crown of your trees and shrubs. The crown is the area of the trunk or stem right above where the plant enters the ground. Putting mulch against this area restricts gas exchange and traps moisture against the bark. Here are a few good websites on proper mulching: