This is not at all a common practice. On the contrary, I read everything Bp. Sanborn writes and this is the first time I have ever seen him append the signatures of other priests to one of his articles.

So, we are agreed it is weird.  What gives?  Perhaps someone will not only confront them about their weak-dog statement, but also inquire as to the weird, arguably-cult-like presentation of a united front (opposing absolutely nothing in any meaningful way, mind you). 

Not only is this statement unspeakably lame, it comes so late in this battle that it would've been better to just remain silent.  So, maybe +Sanborn is getting too old to properly keep up with and address such issues?  That doesn't excuse +Selway et alii, all of whom are just carrying The Don's bags in this instance.  I wish the people at Pistrina Liturgica were still posting regularly; they'd demolish this lukewarm spinelessness for what it is.