My husband asked me a great question the other night "why has our almost 4 year old all of a sudden reverted back to poor past-tense usage?" I had noticed as well that our son, who has extremely good verbal skills for a boy his age, had started using odd word choices for past tense, such as: "like-ed" "good-ed", etc.
The reason I have found after years of working with children as a Reading Specialist, is that at this age children tend to start analyzing the oral language and their usage of it. Beforehand, they were merely recreating the speech patterns they heard around them, but now (around 3.5 or later depending upon the child) they are starting to think about the words they say and the patterns in which they say them. It is very common at this age that they begin to say the "ed" sound at the end of words, or pause to rethink words with improper past tense, such as: flew (for fly), and went (for go).
I find that when my son started to do this I was not concerned, but ecstatic! It means that his brain is moving one step closer to understanding the complex English oral language, and in turn one step closer to beginning to understand that words on paper are connected just like the words we speak.