This is the sudden and unfortunate avalanche-like unleashing of snot as a result of sneezing.
This often happens when you are in a situation where resources to combat said snovalanche are unavailable, i.e in a lecture, meeting, religious service, and so on. Very embarrassing.
You: 'What, they did?! Ah, I had sneezed, but it was a snovalanche, and I didn't have any tissues.'
Friend: 'Oh. Nice. Maybe it's best that they thought you were crying.'