1999 Honda Accord AC Unit repair

Air Conditioning problem
1999 Honda Accord 6 cyl Automatic 150000 miles

Took my car to the shop as the AC unit has been blowing hot air, the car was parked and hasn't been driven for a few months. They performed an AC check on the system to see what was wrong with it. They told me there was a leak, the evaporator, expansion valve and R134 needed replacing (cost $800 + taxes). Asked if my compressor was OK they told me yes, only those parts needed replacing. I was called the day after to say that they replaced the parts and the minute they turned the system on the compressor locked up. What would cause the compressor to lock up? Does the system have to be flushed before starting the system? Is there anything that they might have done wrong to cause this to happen? They are telling me that it is now an additional $1000 + labor + taxes to replace the compressor.