Why do guys spit in the urinal?

It's a really weird question/observation, but all guys do it. We walk up to the urinal and we usually spit while we are undoing our pants and getting prepared to do our business. Some guys spit at the end.

I do it myself, and I can't figure out why. Like I walk up there, and then I get the urge to spit, so I do it, then I'm like why the hell did I do that? I don't get the urge to spit in my toilet at home, this phenomenon seems to only occur with urinals.

Until any real in depth psychological research is preformed, this will remain one of the most puzzling mysteries in the universe.