The subject of what is actually a weather or ATC delay comes up a lot on this board. Just an illustration about how easily a clear cut mechanical delay can morph into a weather/ATC delay:

A few weeks ago, my PBI-BOS MD88 flight, scheduled at approximately 11AM, was clearly going to be late departing as the incoming arrival from BOS had been delayed in departure for mechanical reasons. (Eventually, the defective part was obtained from another aircraft on the ground in BOS and installed in the original aircraft.) So the inbound aircraft arrives in PBI about two hours late and is turned around in more or less the standard time... turns in Florida cities cannot readily be sped up due to the numerous wheelchairs. So we leave for BOS, and on the way, we get a less than 15 minute hold over Providence due to wind/weather in BOS. So we arrive in BOS about 2+15 late... I turn to look at the GIDS for the outbound BOS-FLL flight leaving on that aircraft, and it says "Flight Delay Due to ATC".

So this BOS-FLL flight was delayed over 2 hours almost entirely because the first flight of the day took a mechanical and ATC took the blame because our inbound flight circled PVD for 12 minutes.