That is a cogent observation: pleaded is indeed more emotional (or describes a situation involving more emotion). The change in citations in the media observed and commented on by many respondents can be laid at the feet of the two big style manuals (Chicago and AP) who--for reasons of their own, one must assume--laid down the law about that time.
As some clever wag said, "He pled guilty, then pleaded with his wife to forgive him."
A reporter at a rural Oregon newspaper that carried my syndicated language column ("The Language We Use") back in the '80s raised the question then--and I feel uneasy to this day that I never addressed the issue in my column.
Life in Tropical North Queensland helps assuage any feelings of guilt, so I plead with you not to blame me.