Social networks have started to embrace algorithmic timelines (that is, where posts are sorted based on how the service thinks you will like it), instead of showing each post in chronological order. First Facebook introduced the feature, then Twitter and Instagram, and now Google+ is enabling it for communities.
Google+ already sorts posts in the home feed based on relevance, and now Communities will do the same. Algorithmic timelines actually make sense for Communities, which are often loaded to the brim with spam. Presumably, content that users actually interact with will be pushed to the top, and the spam will be invisible for most users.
This feature is enabled by default, but can be disabled by vising the Community's preferences and turning it off. Google is rolling it out gradually, so you may not see it right away.