Signs 'pop' because they are very effectively retro-reflective (https://en.wikipedia.org/wiki/Retroreflector). The brighter headlights with a wider beam would be casting a moderate amount of light on the sign, which causes it to light up brightly in contrast with the surroundings.

The ones in the UK can be so effective that sometimes I have to switch off my full beams so I don't get blinded by the signs (and I'm not driving a new car with fancy headlights).