A short answer is that it is one of the categorical syllogism rules. A premise that involved a quantifier, such as "some" "few" "many" is known as a particular premise. It is only possible to make a particular inference from any particular premise. Think about it in terms of a Venn diagram, let's say some judges (B) are liberal (C) B some C. The proper inference is that at least some liberal people (C) must be judges (B) at the intersection of two circles representing all judges and all liberal people - C some B. We cannot conclude that most liberal people are judges because we cannot possibly know the quantity of people that are both judges and liberal in relation to the overall pool of liberal people.
Now let's consider an alternative -let's say the initial premise says most judges are liberal, B most C. We can only infer that some liberal people are judges but again we cannot possibly know what proportion of liberal people are judges, hence it would be incorrect to infer that C most B. Also, remember that any "particular" premise can be rewritten with a lower order quantifier, thus:
D most B is equivalent to: D many B D some B D few B
These rules can be confusing and are sometimes easier to memorize but using natural language also helps to visualize the relationship between the subgroups.
