When a member of a group says y'all to the group it is warm.
When an outsider says y'all to a group it can be pejorative.
Compare the following in both senses: "Let's get it together y'all!"
In the first sense, it is a rallying cry. In the second sense, the speaker may think you aren't trying hard enough. There, "y'all" was added only to put some stank on the sentence.
The pejorative may be limited to the end of the sentence.
As in: Think! vs Think Y'all!