While not illegal itself, the practice is often associated with "other illegal business practices", like violating non-compete agreements, or stealing trade secrets.
[1] Employee poaching for the express propose of stealing trade secrets is illegal.
If done for this reason it can give the raider a competitive advantage.
Ethical and legal dilemmas over employee poaching arise from the conflict of interests between an employee's right to free access to the labour market, and an employer's right to protect knowledge and skills which it regards as company property.
[3] Employers may attempt to protect themselves against the most damaging effects of employee poaching by inserting non-compete clauses into employment contracts.