From Wikipedia...
"Post-flight investigation revealed the cable connecting the Left Thrust Lever to the engine assembly had severed shortly after application of Takeoff power. A procedure in the aircraft's Quick Reference Handbook (QRH) titled "Thrust Lever Jammed" would have directed the pilots to shut down the left engine by utilizing the Left Engine Fire push switch, which would have closed (among other things) the Fuel SOV (shutoff valve), thus allowing the speed of the aircraft to be better controlled while landing the aircraft. This QRH procedure would have been accomplished "at a safe altitude" per the procedure, while giving consideration to terrain or other obstacles. The pilots did not perform the above procedure, and instead, chose to immediately return to the airport, landing at a higher than normal airspeed due to the left engine still producing takeoff thrust, making it difficult to stop the aircraft and using much greater runway distance than normal.
The aircraft was taxied back to the Terminal with the left engine still producing takeoff power and the right engine at idle. When the aircraft was brought to a stop at the Terminal building, the right engine was shutdown by the pilots, the left engine was still operating at takeoff thrust, and the hydraulics were all turned "off", which was common procedure at the time. The aircraft disc brakes on the Canadair CRJ-100 are operated by 2 of the 3 hydraulic systems on the aircraft, with System 2 supplied by pumps on the right engine (both mechanical and electric) and System 3 being supplied by two electric pumps. By shutting off the right engine and all of the electric hydraulic systems, the brakes as well as the parking brake were no longer being powered. As the hydraulic pressure bled down and with the left engine still producing takeoff thrust, the aircraft jumped over the chocks, lurched to the right (caused by the left engine thrust), and hit the Terminal building.[7] The left engine was eventually shutdown by the airport crash tender spraying foam into the engine.