A good catch. Did this just happen today (Wednesday)?
The CRJ100/200 is known to shoot out some flames during an engine start with a delayed lightoff (700s and 900s do not do this unless one of the ignition systems is deferred). It is never an issue as long as the engine starts (which is what causes the flame). Basically, what happens, is the starter spins the engine up, at the correct point, fuel and ignition is introduced with the thrust lever. Normally, the igniters inside the engine light the fuel within a second or 2. But occasionally the igniters do not light the fuel immediately, and the fuel spray is shot out the back of the engine as a mist. When the fuel does light inside the engine, the rest of this mist behind the airplane will light creating a large (10+ foot) fireball that sometimes engulfs the entire tail. It is quite a sight to see, especially at night, and usually results in no damage. I suspect that is what the ground controller saw, since that would be about the time they start their left engine.
It is odd that they say they "have smoke indications in the cabin" and evacuate the aircraft, except the pilots stay on board. Flames out of the left engine would have no effect on smoke in the cabin, unless the crew would have started the engine, transferred the bleeds to the engine, and shut down the APU (which would be a VERY rare occurrence with this type of aircraft).