Is reviewing papers moving away from voluntary community service?
I will address your three questions directly, and in-line:


  "Is this a general trend in the research community, i.e. is this
  happening in other domains as well?"


I have never seen this before, and never heard of it. Everyone else that answered and commented, except for one person, has said that they have not seen it happen in their field either. The one person in the comments that did see the trend was Franck Dernoncourt, who says that it's definitely a trend in NLP (which probably means Natural Language Processing, not Neuro-Linguistic Programming). So, it is not a "general trend", but it is happening in some fields (such as yours and Franck's).


  
  "If yes, does this imply a change in the peer-review paradigm? For instance, as a researcher should I stop doing "free reviews" in order to keep my reviewing time for mandatory reviews?"
  


My answer is "no", but even if it were "yes", I do not see how that means you should "stop doing free reviews". The vast majority (by a long shot) of peer-review is still done for free.


  
  "If no, are there other solutions being tested to the problem of ever-increasing papers/reviewers ratio?"
  


I have never seen any other solutions tested. The peer-reviewed conferences I submit to, such as QIP (Quantum Information Processing) keep an extremely high standard for who is able to review. There is a committee of reviewers who are selected because they are recognized world leaders in their sub-fields, and each of them reviews several papers. Forcing any random person that submits a paper to review a paper, would lower the standards of the reviewers, and hence the standards of the conference.