The biology of saliva, phlegm and snot is clear to cyclists: Cycling makes them happen. (Tips et al, Nature, Vol 12, pp.1264-1289, 2011)
The physics are a whole nother matter. Generations of cyclists remain mystified by why - when you spit out into all that fast moving air - it is so rarely whisked away, and so often splattered all over you.
BONUS TIP: The sociology of phlegm splatter could be a whole journal.
When you stop to buy coffee and don't know you have spit all over you, it's ok. Post-ride, when you realise you have been riding for team snot-rocket, emotion surges through you.