Induced currents
RF fields near antennas can be more intense than the radiation field at large distances would suggest, if scaled back by the inverse square law.
Thin conductors nearby can be heated by induced RF currents, and it's also perhaps not impossible for them to cause charge to jump a tiny gap and spark.
Aircraft radios can radiate hundreds of watts, whereas mobile phones produce about one watt. A fire from a mobile phone on a garage forecourt is likely to be urban myth, but I'd say that there is a more realistic possibility that an aircraft radio could start a fire.