The KPD had been (correctly) identified as authoritarian extremists by the social democratic government and threw a shitfit, as under Zinoviev's braindead social fascism theory they saw the social democrats as the primary obstacles to The Revolution™️

Of course Hitler suppressed the KPD when he came to power, so they came running back to the social democrats...until Molotov-Ribbentrop and the party line changed once again. It's worth noting that the DDP, the liberals more properly at the time, were also not the ones tactically allying w/ Nazis.