@jfez You seem to have misunderstood MixBlend.Add. See the documentation here:
http://esotericsoftware.com/spine-api-reference#MixBlend-add
This means that you don't want to set it on track index 0, and when setting it on a higher track you need to ensure that lower track animations already key the same attributes (like e.g. rotation), otherwise your values will be ever-increasing each frame.