probably becasue ur head unit has an amp (ex: 52 W per channel) that is not powerful enough for your speakers (ex: 120 W per channel)...THis is not a bad thing, in fact its fairly common. The distortion sound you are hearing is your head unit's built in amp maxing out. If you want to eliminate the noise, you need to invest in a 4ch or 2ch external amp that puts out the same, or more power than your speakers are capable of putting out. All this imformation assumes that you are using the amp in your head unit and not allready using an external amp.
Speaker distort more often due to lack of amplification, rather than over-amplification. Another reason why speakers distort, is due to its suspension limitations (how far a cone is allowed to move inward and outward, also called Xmax).