In addition to the dastardly conspiracy theories above (unions, contracted pay, on-time statistics, etc.) it's also possible that there was a long take-off line that the plane needed to wait for. Or there was an Air Traffic Control/destination delay or hold, but the plane needed to push back to get in the queue (you can't get a slot in the takeoff line if you're still at the gate, iirc). The pilots may also have noticed some type of anomaly and had to test/verify the problem, but determined it was OK to proceed.