If I had to guess - "political bubble (bursts)" is when an extremely incompetent and/or radical politician suddenly is elected to one of the top posts in a country. So it's like a closed off boiling pot - incompetent radical has 5% of the electoral base and nothing happens, then 10%, then 20%, 30%, 40%, still nothing, and then one day he "suddenly" crosses 50% threshold and bubble bursts, he is now in charge for real.

For example in France their own Le-Trump aka Marine Le Pen had 41% votes last election cycle, so nothing really happened and system centrists won again, politics remained moderate and predictable. But if she or her ideological successor even takes 50%... hooboy, EU will see some Orbanification just like USA does today.

Start with the idea of political capital and end with the monumental loss of value and trust in the political system