It might have been a "hard light." If bugs have built a nest in the flue, gasses can build up in the flue area causing a "no light" on the first attempt to call for heat. After a "purge cycle" that may not have cleared the unburned propane from the water heater area, the second attempt to light off, could have lit off the unburned gas as well as the fresh gas.
If it happens again I would examine the flue for a blockage.
Lou and Laura with Bella - German Short Hair Pointer
2008 GMC Sierra 2500HD Crewcab SB Allison Duramax
2010 Flagstaff 8526RLWS - Superglide 3300
HAM CALLSIGN - KC3FFW