Dust on the opposite side of the three elements glass lens

[thebouljello]

Its been a week since I made a laser with the NDB7875 at 2.4A. After burning some cardboard with it, I noticed there was splash caused by a piece of dust on the side of the lens closest to the diode. I cleaned it off with a q tip and rubbing alcohol but I am just wondering how a piece of dust could have got on the other side of the lens? I burnt paper and cardboard before but never had any dust on the lens that could produce any splash.