These particular mines are called "Limpet mines" from what I've read. They're attached (magnetically I assume) to the hull of the ship as opposed to the floating/anchored mines used in shipping lanes. The only reason I can think of to remove an unexploded one would be to try and hide evidence. I think that is what is suspected in this incident.
That makes a little more sense, I still don't understand how they get attached though, a boat pulls up next to the tanker as it's passing by and sticks a mine onto it?
Also the reason I read to do it was to drive up oil prices?