The thing about bubbles is that they’re fragile. It doesn’t take much—a bit of breeze, a subtle change in surface tension—to burst one.
Before the pandemic, the most common use of “bubble” in a sports context was to describe the precarious status of college basketball teams hoping to make the NCAA tournament. Those on the at-large fringes were “on the bubble”—meaning it could burst at any time, taking their postseason hopes with it. It’s a metaphor for fragility.
Now “the bubble” has become the place where MLS hope to resume its 2020 seasons (not to mention the NHL, NBA and NWSL, which is already underway).