Urinating on others

OK. Here is my question. I have a 5 y/o son who has developed a periodic behavior of urinating on others. First this started on the floor or carpets on various rooms. Now he has done this to his younger brother and both his mother and myself. Additionally, he has begun defecating in his closet. It is periodically by now in occurrence but usually occurs when he is "off" certain days. Such days he would also demonstrate more aggressive behavior towards others, increased difficulty in self-regulation, and some sensitivities more more pronounced.

I suppose my question would be, Is the urinating and defecating behavior related to SPD? Is it neurological or psychologically motivated behavior? Is this an attention-seeking or stimulation-seeking behavior? The defecating in his closet is when he is alone, so should this behavior be looked at separately due to the lack of an audience to supply a stimulating response (i.e., a startled reaction by the victim, etc.)? He usually laughs when peeing on others, but the defecation is when he is completely alone - just as that was in the past when he would purposely urinate in the floor.