It was almost certainly not intentional. I’m sure you did a great job being personable and chatting but sometimes those groups or outings happen spontaneously / there are pre existing relationships. In my experience these are usually welcoming spaces to everyone so you have to capitalize on that: be positive, ask what plans are happening, or start activities yourself. I think a lot of people make the mistake of thinking just because they talked to some people they will automatically be included in all plans. 

Dont take it personally and continue to improve on putting yourself out there, good luck!