Custom post type and custom query, unwanted redirection

Hello !

I have a strange behavior on my WordPress. I’ve created a custom post type with many taxonomies, on which I want to have a multiple criteria search.

So I created a form which lead to the page http://www.campinglorraine.biznetaucoeur.com/?post_types=camping&ville=metz&prestations=3-etoiles+piscine
which work correctly,
but then when I forge the url to be like
http://www.campinglorraine.biznetaucoeur.com/?orderby=rand&post_type=camping&theme=forets-fr-fr&prestations=baignade-plan-deau+piscine-couverte , I’m redirected to /prestation/baignade-plan-deau/?orderby=rand&post_type=camping&theme=forets-fr-fr
which only take into account the frist term of taxonomy “prestations”.

Can someone please tell me why WP is choosing to redirect me to the taxonomy archive instead of just acting in a custom query, and what should I do to stop that behavior ?

Thank you already !