I am a union man myself and having been lightly vilified by certain parties in my own country (USA) i wonder what my life would have been like actually facing persecution and suppression? and what would have been the motive behind that persecution ? i understand that some leaders of the trade unions in this time period would have been communists but what of the rank and file? how would a rank and file member have been treated under the Nazis?
The general reason for the hatred and persecution was the close connection of (most) trade unions, both ideological and organizational, with either Social Democrats or Communists.
Trade unions wielded a substantial amount of power in German politics due to certain guarantees in the Weimar constitution and other legislation. This included the 8 hour work day, freedom of association, collective bargaining and more. On top of that they were able to declare general strikes which was one reason why the Kapp-Lüttwitz coup failed in 1920.
As an independent, generally left-wing and powerful force in Germany they were automatically a prime target for the NSDAP and Hitler's policy of "Gleichschaltung". The leadership of unions that were outspokenly socialist or communist most certainly ended up in concentration camps and the free unions themselves were dissolved and then reformed as the DAF (German Labor Front) and strikes were outlawed. As a rank and file member you wouldn't have been persecuted as long as you submitted to the new regime.