This sometimes happens if your cable is not rated for the environment it is running in and water gets in the POE through the cable. Ive also once saw a faulty switch cause it.
Occasionally manufactures incorrectly rate their cable such as the Tough Cable level 1 product from Ubiquiti.
For tough cable level 1 it turns green cracks when exposed to the sun. After it crack's when it rains water gets in the POE.
They did eventually fix this with the new version that has a slightly different name.
I have used the new version of it at a few installs and it hasn't gone bad yet. (knock on wood). I mostly stick with other brands these days.
Also always use shielded wire with shielded modplugs that are properly grounded. It WILL save your equipment in lightning storms if your grounding is right.
edit: »www.streakwave.com/Itemdesc.asp? ··· SHIELDED
This stuff has worked out the best for me.