I have never heard of normal air combusting under pressure before. Unless they are using pure oxygen or hydrogen as their compressed gas, it's hard to see how the physics of normal air combusting without any fuel source would work.
If you watch that video Tyloe posted above, the combustion effect seem quite long and is clearly noticeable in the video. So it's not like those was a 1 in a million shot catching the missile at the the precise microsecond that flame effect happened.
Taxiya's photo is one that I had found online (from PDF whose image hosting seems to have had issues recently) which shows an S-300 with a similar artefact.