It might be from this:
http://ei.cs.vt.edu/~cs5014/courseNotes/5.TechnicalCommunication/tc_2_Usage.html#Conjunctive
Conjunctive Adverbs and Transitional Phrases Require Semicolons
CONJUNCTIVE ADVERBS: An adverb used to connect or relate main
clauses:
accordingly, also, anyhow, besides, consequently,
furthermore, hence, henceforth, however, indeed,
instead, likewise, meanwhile, moreover, nevertheless,
otherwise, still, then, therefore, thus, and so on.
Example: I don't like the answer; however it appears to work.
TRANSITIONAL PHRASE: A phrase used to provide transition between
clauses or sentences:
as a result, at the same time, for example, in addition,
in fact, in other words, on the contrary, on the other hand,
that is
Example:
Past papers conjectured that the
answer is zero; on the contrary, it is one.
General rule:
Conjunctive adverbs and
transitional phrases connecting main clauses are preceded by a semicolon.