Affix hopping is a morphological operation by which an unattached affix in the
T position is lowered onto a verb. This attachment is done by the "Phonetic Form component" (the posited component in the mind that transforms the inputs it receives from the syntactic component into phonetic spell-outs). For example:
The boy kicks the ball.
The underlying syntax tree for this sentence can be represented as:
Here, the suffix -s (that indicates third person, singular, present tense) has been taken from its original position and then attached to the end of the verb, producing the form kicks.
Head movement is another operation, by which the head of a phrase is moved (in fact, copied) from its original position to another one. This is a syntactic operation. An example of head moving for a question in English:
Will you marry me?
which can be syntactically represented as:
In this case, the auxiliary will has been moved from its original position to become attached to the null complementizer that marks a question.
Both operations involve moving a head from one position to another. But affix hopping is assumed to occur at a different level (or different "mind component") than head movement. What is the fundamental difference that makes linguists classify these two apparently similar kinds of movements as two very distinct operations?