From the character description of the scene its seems like he was aware that something bad was going to happen, because he said that he had the same feelings he had when his Maman passed away. According to my opinion Meursault tried to defend himself from the Arab. In my opinion the Arab was only trying to defend himself by holding the knife against Meursault, earlier in the scene his friend Raymond  got stabbed by one of the Arab so probably Meursault was taking revenge for his friend.