Depending on what you mean by "obvious" :
mu'i zu'i (for the usual/normally implied motive)
This is a nice one. Thanks. I wasn't too familiar with zu'i and I didn't think about using zo'e, although, I don't really like zo'e here... it's too generic (mu'i zoe sounds more like "some motive I don't care to say, for whatever reason")
by obvious reason I meant a reason that I think my listener will intuitively understand.