WordPress messes up with data attributes in shortcode output

I have build a shortcode to insert a call to action text and button/link. The button/link should open a modal window with a contact form. So I added data attributes to the button/link for Bootstrap modal.
Fine.
But the output shows the data attributes messed up. Instead of data-toggle="modal" it shows data="-" toggle="modal" and instead of data-target="#hidden-contactus-form" it show target="#hidden-contactus-form".

Any ideas what could be wrong?