It may not be nefarious, but it's antagonistic to the candidate getting the exploding offer. If you're hiring for a particular role and the candidate is dragging their feet on the offer, you've likely not made a strong enough offer. Making the offer exploding limits their ability to negotiate.

If you've made an offer to one candidate, and you're holding your response on another, you're already doing a disservice to that other candidate. They should know that you've extended an offer to someone else, and that you'll contact them if that offer falls through.