Why do my eyes spontaneously start burning?

Sometimes, especially when I'm stepping outside- but sometimes it happens indoors too- my eyes just start spontaneously burning. I don't know why. My eyes immediately start watering and it's awful.

FAQ before you respond:

1. I wear glasses!

2. I did not touch or wipe my eyes at all!

3. It's not a stray eyelash!

4. I am not doing anything which would cause debris to fly into them. I'm just standing still!