The principle cities are decided based on degree of sociopolitical independence.
I'll use both Austin and Dallas-Fort Worth as examples. The Austin metropolitan area has three principle cities: Austin, Round Rock, and San Marcos. Each of those other cities has a limited degree of sociopolitical freedom within the area, but not enough to qualify as a distinct core separate from the central urban area (I.E. Austin). If there were a substantial sociopolitical difference, Hays county would be considered the San Marcos micropolitan statistical area and would then have to be combined with Austin-Round Rock into a CSA similarly to what has been done to create the Austin-Round Rock-Marble Falls CSA.
The other example is a limited degree of sociopolitical differences greater than that of principle city classification but smaller than that of CSA components classification. This category is MSA division similar to Dallas and Fort Worth. Each city is the core of its own division, but the economic factors and media markets tie them together so substantially that it would be illogical to separate them into two distinct cores, therefore naming them core cities of the MSA and assigning them distinct MSA divisions.
A simpler way to look at this is historic downtowns... Does New Braunfels have a large historic downtown? Yes. It likely then has a degree of sociopolitical independence.
Look back at Austin for comparison. Does San Marcos have an historic downtown? Yes. Does Round Rock? Yes. Do others? Of course, but are the large enough (Georgetown - No) or independent enough (Pflugerville - No) to truly qualify as a principle city? Not quite.
As for the person who asked about the validity of this Wikipedia change? Yes, as a matter of fact, the census bureau did change the status of New Braunfels in a
memo released in early December. The thread I've linked to includes alot of useful info about Austin and a link to that memo.
For what it's worth, I do believe that the Kerrville mSA and the San Antonio mSA should be consolidated into a combined statistical area.