This is pure speculation, but there's two things to consider:

A) It's known that Trump tends to echo the words of the last person with whom he spoke. Kushner is close to him, so we can conclude that he's in a provileged position to talk Trump into doing or saying what he wants.

B) Kushner was the one working alongside with the Mexican and Canadian envoys for the new NAFTA negotiation.

With this in mind, it's likely that Kushner talked Trump into signing something that was good for Mexico -or more likely for the corrupt president and his friends- and he's getting rewarded for that.