For me I see it when I have very ripe pods that the plant was stressed then well watered.
I see it a lot with large podded Annuums mostly.
I also see it in pods I picke that I didn't use right away.
I think the pod was ripe but was trying to re produce before it dried out.
Might be a temperature type thing.Too hot or cool,seeds dry out until conditions are right.
Right temps and moisture,seeds sprout.
I don't find it all that uncommon.
Seems I get it more with thick fleshed pods.
Usually stuff picked ripe and set in a bowl on the counter until I use them.
Annuums and Chinense is where I see it most.
Never in Frutescens ans seldom in Baccatums except in the bigger pods-Aji Amarillo types.
I think it has to do with seed maturity,temps and water content mostly.
Not uncommon,but i used to grow a lot of stuff/1 of 800 varieties in pots...