The DNA in your sample is probably partly aggregated/precipitated and leaches from the well during electrophoresis.

There could be a number of reasons, off the top of my head; the percentage of the agarose in your gel could be too high, DNA carry-over, RNA contamination (if your template DNA was originally extracted from cells).. Also, mis-priming could sometimes cause non-specific annealing and therefore PCR products of multiple sizes.. I would primarily try to purify my DNA, both the initial template and the PCR products before loading onto gel, and decreasing the concentration of agarose.. If that didn't work, then I advise you to go back and review the design of your primers..