Wordpress inserting opening tag inside of my shortcode

[admin]

I have written a small function to display the latest video on my startpage. The function itself works just fine. The only problem is, that wordpress keeps inserting an opening

<p>

tag at one place of the code.
Here is my function:

function video_start() {
// the query
$the_query = new WP_Query(array('post_type'=>'page','post_parent'=>'17','order'=>'ASC','orderby' => 'date','posts_per_page'=>1));
// The Loop 
if ( $the_query->have_posts() ) 
{
    while ( $the_query->have_posts() ) 
    {
        $the_query->the_post();
        if ( has_post_thumbnail() ) 
        {
            $picid = get_post_thumbnail_id($post_id);
            $alt_text = get_post_meta($picid , '_wp_attachment_image_alt', true);
            $string .= '<div class="featured-start">';
            $string .= '<a href="' . get_the_permalink() .'" class="b-link" rel="bookmark">';
            $string .= '<h2 class="mar-bot">Latest Video</h2>';
            $string .= '<img src="'.wp_get_attachment_url(get_post_thumbnail_id($post_id)).'" class="img-responsive" alt="'.$alt_text.'" />';
            $string .= '<p>'.get_the_title().'</p>';
            $string .= '<div class="orange-button">Watch Video</div></a></div>';
        }
    }
}
else 
{
// no posts found
}
$string .= '<div class="clear"></div>';
return $string;
/* Restore original Post Data */
wp_reset_postdata();
}
// Add a shortcode
add_shortcode('video_startpage', 'video_start');
// Enable shortcodes in text widgets
add_filter('widget_text', 'do_shortcode');

The output I am having a problem with is this:

<div class="featured-start">
<a href="http://www.kundenwebseite.inweco.de/videos/pet-project-09-part-1/" class="b-link" rel="bookmark"><br />
<h2 class="mar-bot">Latest Video</h2>
<p><img src="http://www.kundenwebseite.inweco.de/wp-content/uploads/2015/11/pet-project-2009-part-1.jpg" class="img-responsive" alt="" />
<p>Pet Project 09 – Part 1</p>
<div class="orange-button">Watch Video</div>
<p></a></div>
<div class="clear"></div>

I have tried removing the wp-autop filter several ways but it simply wont work. And even after using google for over two hours now i cant find a solution.