There was a crackdown on fraudulent H1B applications, with more RFEs being issued. That freed a lot of spots for legitimate applicants that were hogged by questionable bodyshops. It also incentivized attorneys to start looking at O-1 (and they are easier than people think for real engineers to get).

Academically, a lot was written (by foreign universities) about how the US was going to lose it's edge have a drop in applicants but the rankings didn't change during the last 4 years. I wouldn't be surprised if more people applied simply because they (erroneously) believed that admission would be a little less competitive with less applicants.