My opinion is that dismounting is a prompted action, but I accept that the rules do not explicitly state this. The definition of Prompted Action (page 218) is "All the actions that can be triggered by the playing of cards. All can be done as single UG or block moves.".
As Nik says, playing it any other way leads to a lot of undesirable consequences. Note that it is possible to dismount a block by playing a Yellow and Green card (which again is not explicitly stated in the rules).
Richard