The NHL has filed an unfair labor practice complaint with the National Labor Relations Board. Here is an ESPN.com story. Here's a Washington Post story. At issue is an alleged NHLPA directive that said any players who choose to become replacement players would forfeit the benefits they have received during the lockout. According to this website for the NLRB, an example of union actions that violate the Wagner Act is "Threats to employees that they will lose their jobs unless they support the union's activities." While the union hasn't threatened its members with loss of jobs, they have allegedly threatened them with loss of income.
Another issue that hasn't formally been pursued is the union's telling player agents that they risk being decertified if they represent replacement players. If true, both of these actions are attempts to limit the ability of teams to hire substitute resources.
Reducing the use of substitute labor (scabs) is a common practice in many unions considering that a major component of the elasticity of demand for any resource is 1. the ability for the firm to substitute resources for union labor in the production function and 2. the elasticity of supply of the substitute resources. Replacement players and union players are very close substitutes, so the union would put more effort in trying to limit their availability. The threats to the player agents seems to be an attempt to increase the opportunity costs of becoming a replacement player (I've got to use a non-certified agent, quite possibly a less-talented one, or I've got to represent myself).
If the union can control the elasticity of demand for its labor, that's good for the union. But if it does so illegally and gets caught, that's bad for the union.