Rashi (Bereshis 37:2 sv את דבתם רעה) says:
כל רעה שהיה רואה באחיו בני לאה היה מגיד לאביו - Any evil he saw in his brothers, the sons of Leah, he would tell his father
However it would seem that the plain meaning is that Yosef told his father about the deeds of the children of Bilah and Zilpah who are referenced earlier in the same pasuk. Furthermore, without Rashi's comment the sequence of events would be easier to understand. First the pasuk tells us Yosef spent time with the children of Bilah and Zilpah who were spurned by the other Shevatim and then it tells us that he spoke about them to his father. This could be perceived by the children of Bilah and Zilpah as an act of repudiation by the one person who was seemingly their friend. Hence when the brothers collectively plotted against Yosef it would make sense for the children of Bilah and Zilpah to be in cahoots.
However, taking Rashi's pshat, it would seem strange for the children of Bilah and Zilpah to suddenly turn on the one brother who was kind to them and join with the brothers who acted poorly toward them initially.
What forced Rashi to say a peshat in the word דבתם that obfuscates the simple meaning of the pasuk?