I'd bet economics. There are magic $/experiment thresholds where it becomes possible for scientists to try something, and one the threshold is crossed a bunch of people explore the same area at the same time.
Take neural nets. FLOPS/$ crosses some magic number so that universities can afford the hardware, and bam; AI everywhere. No obvious theoretical breakthroughs since the 70s, but suddenly the field gets intense due to economics.
Take neural nets. FLOPS/$ crosses some magic number so that universities can afford the hardware, and bam; AI everywhere. No obvious theoretical breakthroughs since the 70s, but suddenly the field gets intense due to economics.