I mean if that's true regardless of the picket line stuff its a perfectly understandable reason to sack someone even if you agree with what they are saying. I mean essentially he booked himself to to speak publicly against the guy he works for.
Some people will believe this, others will say he was punished for attending the rallies. Either way it shows that there is still too much infighting within Labour. In a way they are similar to the Tories in that there are quite wide points of view within the party and ideologically they should form their own party. However, no one wants to give up the brand, so they stick with each other even if they disagree. Difference is the Conservatives are in power and have a monopoly on the right, so they can argue amongst themselves without too much damage. Labour can't afford any infighting when it's competing with other left wing parties.