Well, the short and irrefutable answer is "surface tension." However, that's probably not the answer you were looking for...
Sometimes the natural properties of your source water (what impurities are in it) will increase the surface tension of the water. In other cases, over feeding, insufficient filter maintenance, or insufficient water changes can lead to changes in the water that will increase surface tension. As always, I recommend making sure you are providing sufficient water changes and proper filter maintenance and are not over feeding your fish.
Should I be concerned about these bubbles?
The next question you have to answer is, "Are the bubbles a problem?" Some amount of bubbles on the surface of a fish tank are normal, especially in tanks that have air-driven filters, decorative air stones, or have a hanging power filter and a low water line. If the bubbles are popping regularly and are not covering the entire surface of the tank for long periods, you should not have any long-term problems from this. (Though you should still make sure you are keeping up with your necessary water changes and tank maintenance.)
Also remember that some fish, including Bettas, gourami, and other labyrinth fish, will build bubble nests to keep their eggs when they are ready to spawn. Usually the male builds the nest by making bubbles of mucous in his mouth and then sticking them together at the surface of the tank. These bubbles, however, will usually be isolated and controlled - and tended by the nest builder, rather than being loose and floating all over the surface of the tank. These bubbles are also usually smaller than the bubbles produced by an air pump or by the cascading water from a power filter.
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