I read about three possibilities which the SU-27 pilots wanted to enforce:
a) Covering the drone in fuel and then igniting it with the afterburners which should have lead to a fire and subsequent malfunction of the drone
b) Injecting jet-fuel into the piston engine of the reaper, leading to a catastrophic failure of the engine due to misfire. From this source:
With its extremely high flash point, jet fuel, in essence, creates a detonation that will cause a gas-based engine to misfire and eventually fail.
However, it cannot be ruled out that the engine simply would starve due to the fuel-rich air being ingested. Edit after the very good comment of 757toga: The reaper features a turboprop engine. Therefore it would not explode from ingesting fuel-rich air, but it could still starve from fuel-rich air.
c) The SU-27 pilots wanted to blind the optics of the drone by applying jet-A1 fuel.
In the end, the SU-27 pilots chose option d), bending the motor shaft and propeller of the drone by virtue of a direct collision. I do not believe that the fuel of the SU-27 is enough to bend exactly one propeller, therefore I believe a collision occured.
It should be noticed that the exact intentions of the pilots is of course unknown, I also do not believe that they will tell anyone, but they succeded.
Edit 2: The US-military claims that fuel was dumped over the drone, discussion below pointed towards the unignited afterburners could have been used for this purpose. Another theory proposed by JPE61 in his (now deleted) answer, is that they did not dump fuel at all, but merely added power resulting in the sudden onset of a contrail which only looks similar to a "fuel dump". In this scenario the intention of the SU-27 pilots would have been to generate a strong enough wing vortex near the drone to cause either structural damage directly or induce large roll angles leading also to a crash. This is a well known effect (see point 7-4-3. b. in this source) and has led to crashes in the past of smaller aircrafts flying behind larger ones. However, this contrail theory was ultimately rejected...