From the first part, it is so clear that the Bhallaladeva and his father Bijjala Deva hate Baahubali as they believe Power should belong to them rather than Amarendra Baahubali. But the Raja Matha Sivagami who is the Royal mother and Kingmaker makes Baahubali as the King. Jealous father Bijjala Deva and son Bhallaladeva convinces her mother Sivagami that Baahubali is actually trying to destroy their family for Power. Adding more fuel to the flame, Baahubali married Anushka without respecting the word of Sivagami. Convinced Sivagami then orders the loyal servant Kattappa to Kill Baahubali.
However, this is just a hunch, and no one knows the actual truth in it. But the reason is so applicable to the story. Let us wait four more days to know the reason.