My son did this once and he was 22 months old. He learned how to unzip his pj's and take off his diaper. My other two boys did not do this and one was trained at 2.5 yrs and the other was trained at 3 yrs. So, I don't think it is a signal to start potty training. I just think it is a signal that they know how to undress themselves. When it happened, I researched the Mamapedia site and found a GREAT solution: Cut off the feet of the pj's and put the pj's on backwards. I put him in a foot-less blanket sleeper for nap and overnight and I have not had any further bm exploration problems.
Also, don't stress over whether or not this normal or abnormal behavior. Every child is different and offers a different set of challenges to their parents. If you do not have a poop exploring baby, then great...but you may have a child that has another difficulty which puts you at your wits end. Keep everything in perspective and smile through the good times and breath through the bad.
Go buy some blanket sleepers, cut the feet off and put them on backwards. Good luck!