I think the various seals can make a site look spammy and less trustworthy.
It's like going to a B&M business. Would you prefer to do business with the company that looks neat and organized or the one with a bunch of clutter all over the place, even if awards and trusty memberships are among the clutter?
Clean and simple trumps clutter.
Those trust seals just give the potential customer one more distraction from making a purchase. If your site emanates trustworthiness without trust seal, that feeling of trust is in the potential customer's subconscious mind. By then hitting them with a trust seal, you bring the matter of trust into their conscious mind. In so doing, you are giving them a reason to consider not trusting you rather than the other way around.
And that can explain a drop in the conversion rate.
Conversely, I agree with others that if a trust seal increases your conversation rate, there is likely something else about your site that the trust seal compensates for. It may more cost-effective to try to figure out what was wrong with the site and fix it, rather than to spend extra money compensating for an unknown defect.