pulling away from the gate and then sitting there 47th in line to take off), crew members being late (either because they are connecting or because they simply got to work late), unexpected maintenance on the plane (inside and out), delays with the food catering team, delays in the cleaning of the plane between trips etc??

Then there is a ripple effect... If a NYC to SLC flight in the morning is late, that means that the 50 or so passengers who have connecting flights are also delayed... so what do you do if you're Delta? Make them all miss their connections or delay some flights so people can make their connecting flights?