It's due to drainage.......poor drainage is the primary reason bulbs rot. It is quite possible that some areas of your garden drain better than others and it is also possible that excessive or prolonged shade reduces the normal evaporation of soil moisture, leaving shady areas damper than those that receive more sun.

I have them planted in a raised bed that gets shade in the summer from a branch of a honeylocust tree. The shade must have made it damper than if it was in full sun where moisture evaporates quickly. Thank you for your explanation. That makes a lot of sense.