The question wasn't really "Why do same-size bubbles form a regular pattern?" - it seems to me fairly obvious that they would do so when drawn together by surface tension. The question is, "How did all the bubbles come to be the same size, and why does it not happen more often?" (it wasn't by someone experimenting in the sink).
My wife and I have a theory each:
- There were originally a mixture of sizes, but those of a certain size are stronger and longer-lasting and after a while the others burst or dissolve.
- Vibrations from the nearby washing-machine have caused standing waves and an interference pattern which formed the bubbles separately in a grid. After the vibrations stopped, surface tension has drwna the bubbles together.
I think the first situation (and a few other possibilities) would be far more commonly observed than this. The second proposition would probably only occur under very specific conditions (depth of water, objects in the water, other damping or amplifying effects), which are difficult to reproduce.
