Sorry guys, just realized that it might be the theme I'm using. So I googled a bit more and and thought I'll post my solution here for someone else. Ended up doing this to override the default textbox theme.
<stackpanel.resources>
<style targettype="TextBox">
<setter property="VerticalContentAlignment" value="Top"></setter>
</style>
</stackpanel.resources>
The only issue is that now the textbox isn't styled like the others, but this is fine for what I'm doing.
Regards