vertically growing icicle
It pretty much has to be a supercooling phenomenon.  This page from the University of Toronto Physics department presents an explanation that I think fits the scenario in the question nicely.  The description and photos used suggest that this is likely the case for this question:


  The short explanation is this: as the ice freezes fast under
  supercooled conditions, the surface can get covered except for a small
  hole. Water expands when it freezes. As freezing continues, the
  expanding ice under the surface forces the remaining water up through
  the hole and it freezes around the edge forming a hollow spike.
  Eventually, the whole thing freezes and the spike is left.
  
  A slightly longer explanation: the form of the ice crystals depends on
  the cooling rate and hence on the degree of supercooling. Large
  supercooling favors sheets which rapidly cover the surface, with some
  sheets hanging down into the water like curtains. These crystallites
  tend to join at 60 degrees and leave triangular holes in the surface.
  Hence, spikes often have a triangular base. The sides of the spike are
  sometimes a continuation of pre-existing subsurface crystallites, and
  can extend from the surface at steep angles.


Notice the triangular shape at the base of the ice spike in the photos in the question, as is also mentioned in the description above.  The spike also appears to possibly have a hollow region through the center.  Additionally, the flexible plastic lid would allow for some pressure buildup before the release through some weak spot in the ice sheet.