A strike involves work stoppage, it's a very different tactic than mass resignation.
In 1999, Major League Umpires Association (MLUA) were not able to legally go strike so they opted to use mass resignation as their negotiation strategy. It backfired, cost 22 umpires their careers and led to decertification of the union.
Sickouts are commonly illegal. Though it is usually impossible to punish the individuals for calling in sick, an organization that promotes it - such as union - may suffer consequences, and organizing such thing may be a firing offense.
In 1999, Major League Umpires Association (MLUA) were not able to legally go strike so they opted to use mass resignation as their negotiation strategy. It backfired, cost 22 umpires their careers and led to decertification of the union.
http://en.wikipedia.org/wiki/1999_Major_League_Umpires_Assoc...