The full quote is, "Small-cap stocks – which continue to drag along like road kill caught on the rear bumper of the market – are our main cause of concern at the moment."
Road kill is an animal lying in the road that has been struck by a car. If the animal is caught on the bumper, then it will be dragged along with the car.
The phrase refers to businesses that are doing poorly in the current market, but are not quite bankrupt - they continue because the market continues, but do so with the vigor of a dead animal. One more bump, and they lie motionless on the road while the market drives on.