I have a ATC question-

Me and my dad droped off my mom and brother at TPA today at about 1 o’clock and they then departed at 3:50 p.m to Toronto, 20 minutes after scheduled departure time. Flight AC902, from YYZ, had run late b/c of heavy air traffic up and down the U.S. east coast. This traffic lasted all day, and as a result, AC903 was late by 40 minutes, arriving at YYZ at 6:58. My question is when there is a substantial amount of traffic in the air what does ATC do to organize it all? The original departure from YYZ, AC902, departed on time so how did they arrive late? Obviously, im thinking that their routing was longer than usually used or maybe they were put in holding patterns around the airport TPA, but then again TPA is not exactly busy.Can someone clarrify this for me? Wys, WD, Skycruiser?

Sorry about the bad grammar, but i would have to rewrite it all to fix the mistakes.