Moving pipe changes the pressure at the bottom of the well. Pulling out swabs the hole and can bring on a kick if the mud weight has little margin over pore pressure. Running in surges the hole and can start losses if the circulating density was already close to the fracture gradient. A round trip is out and back in.
The hole has to stay full as steel comes out. A trip tank is used so the mud volume that replaces the pipe can be compared with the steel that was removed. A hole that does not take the right volume is either swabbing or losing fluid, and the trip stops until that is understood.