Generally, you only get burning patches if you use: a. The substitute powders, that burn at a much higher temperature than does BP; b. large quantities of FFFg powder behind a thin patch( Usually thinner than .015"). It seems that once you load more than 70 grains of FFFg in any barrel, you chance seeing burned patches, particularly if they are .005, or .010" thick.
Crisco seems to melt somewhere between the boiling point of water( 212 degrees F. and the burning point of paper( 451 degrees F. Since BP ignites about 460 degrees, I suspect crisco is melting or burning up as the PRB moves down the barrel, and is consumed on those THIN patches. That leave the patch itself to catch on fire.
If you use a thicker patch, it holds more grease, and the patch makes it out of the barrel and cools in the air to keep from burning, before all the grease is consumed.
You might pay better attention to barrel length, caliber, powder used, and load used, in addition to patch thickness and how the Crisco is put on the patching in the future, to figure out the true source of the problem of burning patches. A folded cleaning patch use as a fire wall between the powder and the PRB is an easy solution. ( it also gets rid of those dirty cleaning patches!)