I have no idea how likely it is general, but it seems that flares are indeed sometimes blamed for causing wildfires on the ground.
In New Jersey (2007):
A massive wildfire that has already burned thousands of acres in the
Pinelands and forced the evacuation of residents in two towns likely
started this afternoon when an F-16 fighter jet dropped flares as part
of a maneuver over a gunnery range, New Jersey National Guard
officials said tonight.
"We believe it was one of our F-16s on a routine training mission that
dropped the flare that started the fire," said Lt. Col. James Garcia,
a spokesman for the state Department of Military and Veterans Affairs.
"There's going to be an investigation."
In Oregon (2017):
A remnant of a military flare was found at one of the areas burned,
triggering a call to the Air National Guard, which uses flares in
self-defense training exercises over those same areas that burned.
Flares are deployed in air combat to draw heat-seeking missiles away
from their targets.
After being contacted by investigators about the fires, the Oregon
National Guard agreed to suspend the use of flares. Agencies also
engaged them about mitigation techniques to employ if it is determined
that the guard was the cause.