I'm no expert, but I'm sure it's because:
1) It uses mein-. Therefore a mixed inflection 2) it's accusative and the gender is neutral.
here is a good site for the adjective endings http://germanforenglishspeakers.com/adjectives/adjective-declensions/
Watch this playlist
It's seriously amazing at demistifying German grammar pretty quickly. To answer your question, it's because of it's Genitive case (possesive)