What happened in 2007 that the refusal rate for Mexicans grew from 10% to 33%?

Spain's rate went 11% higher too after 2011. Was there any policy change?

The case of Spain is not highly representative since B types visas are ordered in extraordinary circumstances. Spanish people is allowed to enter USA by having an eVisa: https://en.wikipedia.org/wiki/Electronic_System_for_Travel_A... About your exact question I do not know why that difference in 2011 and the following years. It would be interesting to know.

I only found that in 2011 the ESTA fee was introduced but I do not know if there were more changes in the visa policy.

In 2011 517 visas were issues to Spain, in 2013 it was 469. I'm sure if you look at further years it was simply more people applied for visas in 2011.