Indeed, it is posible.
Retro pvp works different from other kind of servers. Using runes on the ground will attack people in its area even when you have not selected the pvp mode.
Same works for field runes; during the first ~10 seconds after you use them in the ground, if someone steps on the fields, you will take skull, protection zone lock, and frag if this person dies. After this time passes, it is safe even if someone appears.