One engine carrying four firefighters arrived at the blaze, on Wildridings Road, at about midnight.
The fire took 20 minutes to extinguish and firefighters left the scene at about 12.20am.
Watch manager Miles Cooper said: "It could have been started by someone flicking a cigarette under the underpass or it could have been malicious. I'm not sure.
"I was very surprised to find a pile of dry leaves."