You may have moisture in your solder paste which is causing the solder balls to explode off during reflow. This can happen if the paste is refrigerated and not allowed to get to room temperature before opening causing moisture to condense on the paste.
There are a number of factors that can cause solder balls or solder splatter. Moisture being one which you have eliminated by baking. I recommend verifying your under stencil cleaning process to make sure the stencil is not depositing solder paste randomly around the board. Also, make sure your preheat (ramp-up) is slow enough to allow solvent to vaporize and out gas gradually.
You may also want to see if this is occurring on all PCB types or just this one. If you run multiple products, you can narrow your variables down by running other products with the same paste and equipment to see if they exhibit the same problem. Good luck, I hope this helps.