There's a microeconomics concept that says: when a particular company in an industry uses a technology or tactic that improves their business economics, all other competitors must follow suit and adopt the profit-improving technology too. Otherwise, their margins will dwindle due to pricing pressure from the innovator and they'll be competed out of business.
When I watch political debates, especially within the primaries of a political party, I feel a similar force is at play. But instead of it being a new technology that must be adopted, it is an effective talking point that somebody utters first. If one candidate figures out it is politically advantageous for them to say, "I am fully in support of [insert populist idea here]," then many other candidates will realize the same, and say the same until all coincidentally hold the same populist positions.