If another business/company were to hire contractual employees (that were still under contract) from an employment agency, what is the violation?

If the employees were still contractually bound to be employed by another employer (which would be a little unusual; most employment agreements, except for those at the higher levels of the business, are terminable at will), they would breach their contracts by becoming employed elsewhere.  If the new employer hired them knowing that they were contractually bound to be employed elsewhere, it would have committed tortious interference with contract, and be liable to the first employer for any damages sustained from that wrongdoing.