It's a gray area to some extent, where benefit of the doubt is in order, however there is the undeniable logic of statistical variance.
When a three year old document, that never attracted any ratings or comments, now suddenly receives almost a dozen of these within a few hours (and obviously all from workmates), then something "stinks". That was actually the trigger that made me check further and find all the patterns as described in Jürgen's excellent reply. Of course I can only speculate about the motivation, internal KPIs based on SCN points, overall lifting of a company's reputation, beating the system, just for fun, ...
Play according to the rules, fairness and common sense, then there will be no issues.
Thomas