Loop para adicionar uma classe ao primeiro elemento.

Php Init WP

<?php $firstMarked = false; ?>
       		<?php if ( have_posts() ) : while ( $the_query->have_posts() ) : $the_query->the_post(); ?>


			<dt class="collapsible-title <?php echo !$firstMarked ? "is-active":"";?>">

				<h1><?php the_field('titulo1_video'); ?></h1>
				<h2><?php the_field('titulo2_video'); ?></h2>
			  </dt>
			  <dd class="collapsible-content">
			  	<?php if(have_rows('videos_youtube')): while(have_rows('videos_youtube')) : the_row(); ?>
					<div class="video-yt">
				    	<div class="embed-responsive embed-responsive-16by9">
				    		<iframe width="560" height="315" src="https://www.youtube.com/embed/<?php the_sub_field('link_video');?>" frameborder="0" allowfullscreen></iframe>
				    	</div>
			    	</div><!-- /video -->	
				<?php endwhile; else : endif; ?>
			  </dd><!-- /collapsible-title is-active -->
			<?php $firstMarked = true;?>
			<?php endwhile; else: endif; ?>