Salman is known to have very strong likes and dislikes. If he is a friend of someone, he goes out of the way to help him/her if someone rubs him the wrong way, he can be very vindictive on him/her. I guess Shakti and Sallu had a tiff and for that Salman would not have wanted to talk to him and hence Sanjay could not talk to him as well. As far as the comment goes, filmi people, well a lot of them, goes beyond the line to make fun of others.
About background of all three Shakti, Salman and Sanju all have dubious pasts better not to bring it here.