tl;dr try to long tap on them and tap "Remove".
If you are talking about the left side panel, the communities are grouped into two groups: bookmarks and subscriptions.
Bookmarks are entirely client sided and are much more powerful. They are unique to Summit and are per user (as opposed to per account). Bookmarks are supposed to be a more flexible way for you to curate your feeds. Eg. you can bookmark special feeds such as your subscribed feed. You can also bookmark mutli-community feeds (eg. you can group 3 communities together and have them show up as one feed).
Subscriptions are server sided. They follow you on other apps or on the website.
In both cases these are added by the user. They are not auto-populated by the client. Maybe you accidentally tapped something? To remove a bookmark you can long tap on one and tap "Remove".