What can be inferred and how common is that a potential PhD supervisor talks directly to my recommender? I would like to ask about a reasonable interpretation that can be made in the case that an aerospace engineering faculty member of a high-level US university, like Caltech or MIT, directly talks to my current university professor about my backgrounds and preparations before my formal submission of application.

Actually, I have been in contact with a faculty member since two years ago and I provided him periodically with updates regarding my work, master's dissertation and publications etc. The professor constantly expressed his interest in my background and even wrote in his last email that "I very much look forward to read your PhD application". A few weeks after this last email, there was a conference that my thesis advisors, one of whom is prominent in my field, were attending and the faculty member talked to them in person, said that he will certainly read my paper, which was published in that same conference, and that he is indeed interested in supervising my PhD. 

Of course, I applied to the program and I am now waiting to hear back from the admissions committee and I read in some other questions that in top-rank institutions, the decisions are typically made by a group of faculty members, not necessarily the potential supervisor alone. 

As such, I thought to ask this question so that it would be possible to find out how common this kind of situation is and what can be reasonably understood out of it.
Sounds like a polite chat. Professors are humans. They go to a conference and meet all kinds of people they don't know well and they need a topic to talk about, and a common acquaintance is a good topic for small talk. It is a bit tricky in the sense that there is an application involved here, but overall I would not over-interpret this.

If you want to interpret this, then you can see this as a slightly positive sign: (S)he is also signaling that (s)he is interested in supervising you. That helps: if one committee member says that (s)he wants to supervise a potential students, then there needs to be a good reason not to go along with that (e.g. lack of funding).