You drop a dwarvish mattock. Your movements are now unencumbered.
You see here a tin. KAABLAMM!!! You triggered a land mine! Your movements are slowed slightly because of your load. You fall into a pit!
Why did this happen? I picked up some scale mail, and became burdened. So I dropped my dwarvish mattock (I still have a pickaxe). That un-burdened me. I walked over to a tin I saw and stepped on a land mine that was suddenly there! I fell into a pit, and then somehow I became burdened again. The tin, meanwhile, flew across the room. (Poor tin.)
How did the explosion increase my weight? Is this a kind of status effect? If so, how could I remove it? The wiki article on land mines mentions nothing about this, and I don't have autopickup on (the tin flew away anyway).