It is quite bizarre. I noticed this, yes about 5 years ago, as you say. What puzzled me was how it became universal in the media almost overnight. No one said "he pled guilty".
"He pleaded", as far as I am concerned has a completely different meaning, inferring that the person was in imminent danger and was asking to be spared from imminent harm.
There are two questions; how this happened and why?
My gut feeling is that there are certain websites (often American) that give advice on use of English and the solitary advisor for the site just chose pleaded as the best option for most purposes. Obviously I disagree but the younger, more easily influenced media types just fell into line.
Today though, I noticed another similar change; speeded instead of sped. Should we worry? Am I annoyed? Well, yes and yes, but clearly people like me or sites like this one don't have the influence to counter the behaviour of media types.