Sunburst was visiting ponyville because of a task Twilight gave him. He was pissed sense he was working on his research before he was given this task. So he definitely was in a bad mood. It has been a long time sense you have seem Sunburst, your partner, so you went straight to visit him when he got to ponyville. He was in annoyed, definitely not in the mood to talk. When you walked up to him to welcome him, he walked pass you like you weren’t even there.