Make bootstrap dropdown menu go only downwards?

Question

i have a little problem with my Bootstrap Select Box on the following

Website

As you might notice while scrolling the page up and down its disturbing the dropdown opens to both sides up/down.

Now i am not sure if this might be a bug or standard functionality of the Bootstrap Select.

Preview:

How i am able to fix this to only opens downwards?

thanks for your kind help in advance.

Solution

Options can be passed via javascript

$('.selectpicker').selectpicker({
    dropupAuto: false
});

or

by HTML tag

<select class="selectpicker" data-dropup-auto="false">
    <option>1</option>
    <option>2</option>
    <option>3</option>
</select>