Had a squib today, not sure what caused it

I had a squib load in my Mosin just now. I'm always careful with my powder charges so I doubt it was an undercharged case. I just switched to a Lee turret and I use the double disk kit, bit since I still check each round I don't think that matters a whole lot.

I don't have any pics but the case looked fine on inspection and the primer was struck fully. You wouldn't be able to tell it was a squib just by looking at the case itself, minus the fact that there was unburned powder still in it.

My friend was the one shooting it and he said the trigger clicked and powder granules came out everywhere. I noticed a good amount on the shooting table. The power was Win 748 and I throw between 44 and 45 grains. Not sure what the actual charge is offhand but the powder throw is very accurate.

Now for the interesting part. There was a mass of caked together powder which was just large enough to be stuck in the case when I tried to shake it out. Also, the powder had its typical grey/black color along with some yellow thrown in, which I have never seen before.

My AR was using the same powder and fired just fine, and we shot about ten rounds through the Mosin before the squib happened.

I can't think of what the cause may have been. My guess would be the temperature/humidity caused the powder to degrade but I can't imagine that would happen unless the situation was severe.

The humidity was pretty high since we're expecting a thunderstorm tonight, and the temperature was in the low 80s.

Any thoughts from more experienced loaders out there?

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