This is just a draw out of many games, I don't think this is the start of a declining run for Arsenal. Just because they lost two points doesn't mean Arsenal will lose the rest of the games ahead. On the contrary I think the team will bounce back in the next game and get a win to stay at the top of the table. It was a tough game for Arsenal against Southampton and I'm not surprised that the game ended in draw
All teams lose points sometimes it's inevitable (even City have lost 2 points more than Arsenal at the moment) and it's impossible to predict when it will happen. In the next round, Arsenal play with Nottingham Forest - the worst team in the league, which it is ahead of by almost 20 points. Most likely this game will be very convenient for Arsenal to regain their confidence and score a lot of goals. Nottingham Forest concede an average of 2 goals per game and Arsenal have the second attack in the league - a big score in this game is extremely likely.