Well Control - Loss of Primary Control
There are four principal causes of loss of primary control: swabbing, insuficient fluid density, losses, failure to fill the hole correctly.
Swabbing can also be caused by pulling full gauge tools which restrict the passage of fluid in the annulus. This creates a piston effect and the volume below the tool would be filled by the fluid inside the drillstring, causing a large drop of the fluid level in the drillstring and consequent dangerous reduction in hydrostatic head. "High Volume" swabbing is especially dangerous in large diameter holes.