Chelsea had a decent beginning, leading by two goals, but a lack of experience in certain positions, such as Robert Sanchez's, allowed Arsenal to level the score. I believe Chelsea could have won this match if they played with more experience. They handed the game to Arsenal when Pochettino reinforced the defense while they were only ahead by one goal, which seemed quite unreasonable.
Chelsea could have have also extended the lead but they are not clinical enough in front of goal and that was why Arsenal was able to punish them and secure a point. Chelsea was excellent in midfield and defense but the goalkeeper and attackers cost them the match. Sanchez could have cleared the ball away but chose to be unfortunate, his performance was bad and couldn’t have any decent save in the match. I think pochettino should query him and make him understand he can’t keep making such mistakes.
Chelsea not been able to defend their goal at home to secure a win was really disheartening, pochettino mad a mistake when he removed plamer and other in the later part of the second half, he changed to a defensive tactics maybe to protect the two goal he had already but it was wise enough to weaken the front line and strengthen the defence, as Arsenal to advantage of the less pressure on the attack.
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.