-The psychodynamic model assumes that all mental disorders are caused by unresolved psychological conflicts that occur in the unconscious part of the mind.
-It explains phobias as originating from early childhood experiences where our urges clash with what is 'acceptable'.
And then to go into more detail, it explains phobias having developed as a result of using the 'displacement' defence mechanism to cope with anxiety from the Oedipal complex (which is where you have a desire towards your mother lol).
So basically when you use displacement, the anxiety caused by the stimulus of the oedipal complex is shifted to another stimulus which becomes your phobic stimulus.
Sooooo, if a boy had a strong desire for his mother, he would be like "oh shit this is so weird, what happens if my dad finds out and bashes me" hence causing anxiety. So he moves this anxiety onto say, a spider, and creating arachnophobia. (You can also say that spiders resemble the father, hairy legs = hairy spider haha)