I suggest not putting too much emphasis on this one factor. The reason is that there are just about as many reasons for this as there are associate professors. You've named a couple of them. Let me add a few more.
Some recently tenured faculty need a break, especially if their path to tenure was especially arduous.
Some recently tenured faculty want to change their research focus now that they have the freedom to do so. This takes a while and may seem unproductive from the outside.
Some people contribute collaboratively to the work of others and are happy to let the others take the major part of the credit, especially since their own position is now secure.
In some collaborations it is difficult to say who was the "major" contributor. As you note, some professors are happy to let their students take the lead.
Some professors just aren't enamored of the "paper chase" and are happy to do research for its own rewards.
But if they are leading a lab, and the lab as a whole is productive, they might be ideal advisors. They aren't as likely to step in front of you at the last moment claiming first authorship. I'll guess a lot of "first authorship" by professors is demanded rather than earned. I'm relying for that on questions asked on this site, rather than personal experience, however.
I contributed quite a lot to a lot of the projects of others and was happy to do so. I was secure in my position and didn't need any citation count heat index confirmation of that. The work was good and made contributions. I doubt that in most of it anyone really thought about authorship order (one exception where it was very clear - to all of us who had the lead). In particular, I don't appear on the dissertations/publications of my doctoral students (other than an ack), nor did my advisor ever consider joining me in authorship on my work, though he did contribute to it.
In choosing an advisor, think more about who can be helpful in the short and long term. Your reputation is your own, it doesn't derive from that of your advisor. Ask around (other students) to see what sort of support you are likely to get.
I had a friend on the faculty (tenured) while I was a grad student. He predicted at the time that he would never make full professor. It turned out to be too pessimistic.