The Flash Point of a chemical is the minimum temperature at which the liquid produces a sufficient concentration of vapour above it that it forms an ignitable mixture with air.
Propylene Glycol has 2 -OH bonds. It is capable of forming H-bonds with water. Since the attractive interaction betweeen propylene glycol and water is stronger than the attractive interaction among propylene glycol molecules, then the vapour pressure of propylene glycol should be less than it is in the case of a pure sample.
A lower vapour pressure translate to a lower concentration of propylene glycol vapour in air, therefore a higher temperature is needed to ignite. This means the flash point increases.