A common strategy to mitigate risk is to short a stock you...

A common strategy to mitigate risk is to short a stock you already own. So example. Lets say you own 10m shares are worried about a potential fall but also you don't want to sell as it may be hard to re-build a position. So you take out a lesser short position (say 5m) to hedge part of your holding against a possible fall. Why not hedge the whole thing you ask... Well in that case you would negate the entire risk ...but why not just sell out the lot which achieves the same. Its possible.

So have a long and a short at the same time is common in options trading with various strategies to numerous to name. CFD will provide you we an easy tool to do this only you have to pay daily interest on the short position as a cost of doing it.

Most likely its a trader with a one sided position IMO just trying to scalp a profit ...but their may be some hedging in the mix also.