IMO, the administration thought they held the stronger leverage when it came to soybeans. They knew their increase of tariffs would at some point provoke the chinese into retaliation against imports of soybeans. With the US accounting for a large percentage of total  chinese imports and having basically only the US and Brazil as primary suppliers, I believe the US felt that, in terms of soybeans, they held more leverage than the chinese did. 
 
 But there are more larger issues at stake than just soybeans. I believe that trump, being a populist, is really doing what he believes to be right in the long term for his base,( and for America as a whole) including farmers, only time will tell if he right or wrong.