Ok, here’s my theories.
1- Rebecca and Jack were out somewhere watching the Steelers play (maybe another Super Bowl?) Either Kate or Randall call to say the house is on fire and Jack races home before Rebecca to save the kids..something happens and he dies in the fire or at the hospital. The kids are with Miguel, because as another poster said, they had to go somewhere during/after the fire.
2- Rebecca goes out alone to watch the Steelers play. Jack stays home because he doesn’t want to be around the alcohol. Something happens and a fire breaks out..cue the rest of my theory.
We learned that the kids were 17 when Jack dies. Matching to their birth year, did the Steelers play in the Super Bowl the year they were 17?
ETA- I’m really starting to like Toby. I sympathize with him feeling like the third wheel with Kate and Kevin. My ex husband was like that with his sister. He even called us the same pet name

The first time I heard it was the last time that it happened.