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Perhaps the air bubbles in the ice formation allows floating to take place!
This may be the old "nuts in lager" phenomenon, (no, not a rugby club initiation  )If you drop a peanut into fizzy lager it initially sinks to the bottom where it collects bubbles, then after a minute it floats to the surface, it then sheds its bubbles and sinks back to the bottom of the glass. The cycle repeats itself with the peanut (or raisin) rising and falling until the lager goes flat.
They may have frozen to the bottom of the glass. There would be a bit of melting when the ice cubes first touch the glass but as glass is a good insulator and water is quite a good conductor, it's possible that the ice cubes are able to transport the heat away from the glass more quickly than the glass can conduct more heat to the ice, so the water re-freezes, sticking the ice cubes to the glass. Eventually, the ice bonding the cube to the glass melts and the cubes float free.
If the ice cubes are stacked on each other like in the pic, then the weight of the top cubes will hold the bottom ones down, even though they are trying to float. Once the water level rises up to the top then all of the ice cubes will float.Ice is less dense than water because it expands when it freezes, there is no need for air bubbles or fizzy drinks.
I see, LeeE's suggestion seems most plausable then