Without looking over your shoulder it’s hard to tell. That said, the most common reasons that come to mind are:

  1. a dull blade
  2. trying to make too tight of a turn, too quickly
  3. trying to cut green or wet wood
  4. incorrect blade for the cut or material to be cut
  5. improper tension set on the blade
  6. trying to cut faster that the wood will allow

When all else fails, read the owner’s manual that came with your saw.