More frequentely, the acquirer of a company usually has a major stock decline, specifically a hostile takeover where there is a bigger gamble for success for the investors and share holders.
Being that U wants to shell out $10 billion paying about $4 billion more than DL is worth, will drop any stock.
I suspect that the more it appears the deal may go through, the more acute decline in the stock value you will see.
On the other hand, being that oil prices have been droping, I would think that the entire industry has had a bit of a run.