SO! That's WHY?

      Aft er the continuous deluges we seem to have had in the last week or several, I've been thinking - "Can't have been THAT bad, ever time I look, there's only a fraction of an inch." 
      Today  I thought I'd better lift it out  & empty it anyway.  Which of course was when I noticed "IT".
      Question?  Was it Willy WindBounced something off it? Or frost? Can't imagine it  being frost, for two/three reasons:-
      1. Most of winter I emptied it.
      2. It never got anywhere near half full.
      3. Due to the tapered shape I'd have thoughtIce wold have slid up rather than bursting.