Collins (astounding that they would have a term like this) renders it as baja colectiva por enfermedad como forma de protesta. It seems to me that baja, in context, implies the por enfermedad part. Is it not also obvious that if it's being done colectively it's as a protest? Maybe not, but certainly in simultaneous I would just use baja colectiva.
On a related note, a walk-out could refer to a wild-cat strike, but the term is also used for student walk-outs, for which I would just use huelga.