Solder shorts during reflow soldering occur for a number of reasons the most common being too much solder paste or more correctly too much metal. When examining reflow problems, consider other causes, poor solderability of the leads or pads not allowing solder to wet as far as on previous occasions. Movement of the component during reflow due to poor control of convection rates or vibration in the conveyor.

A common cause is incorrect reflow profiles causing shorting. If you try watching a part reflow you will often see shorts forming initially as the paste slumps on fine pitch parts. In the liquid state shorts will break as wetting occurs when the solder is being stretched between the two pins. If the board assembly starts to cool at that point a short will remain in place, hence it may be a profile issue.