They were comfortable until the blunder from Sanchez, before that mistake Arsenal was not much of a threat and i even wondered how they could score but a 2-0 lead is very dangerous when the losing side gets a goal back, they would be more determined to level up and that was exactly what Arsenal did they capitalized on that goal.
He made those changes because he felt the game was secured already and it was what every reasonable coach would do when you are 2 goals up, you would want to defend it until the end. He was just unlucky to have conceded the first goal earlier.
You're right, Chelsea were too comfortable, they got carried away by the 2-0 win, if they had defended properly and tried to add more goals I believe they would had won the game, to be honest the 2 1st half goal really killed the morale of Arsenal players but Chelsea gave them a better opportunity to score one and it lifted their team spirit that was why they were able to get a comeback draw.
Pouchettino is really doing a good job currently but I think Chelsea's defense needs more strengthing, they should try strengthen their defense line by the January window, well the both teams put up a show yesterday and made the match so interesting, it was the toughest match of the week, Arsenal were lucky to get a draw they were the favorites but didn't quite impressed in the 1st half and conceded 2 goals well it's a London Derby so things like that are always expected.