What phenomena was this video from SOHO actually showing?
It's a polar crown cavity or coronal prominence cavity. Here is another Youtube video explaining it. It references A new look at a polar crown cavity as observed by SDO/AIA


  Conclusions.
  We propose to define a polar crown cavity as a density depletion sitting above denser polar crown filament plasma drained down the cavity due to gravity. As part of the polar crown filament,
  plasma at different temperatures (ranging from 50000K to 0.6 MK) is observed at
  the same location on the cavity dips and sustained by a competition between the gravity and the curvature of magnetic field
  lines. The eruption of the polar crown cavity as a solid body can be decomposed into two phases: a slow rise at a speed of 0.6 km/s,
  and an acceleration phase at a mean speed of 25 km/s


The video on this page shows one forming nicely.