MacAlpin's treason is a medieval legend which explains the replacement of the Pictish language by Gaelic in the 9th and 10th centuries.
The legend tells of the murder of the nobles of Pictavia (situated in modern-day Scotland). Kenneth MacAlpin's mother was probably descended from the royal house of Fortriu and his great-grand uncle, Alpín, had reigned as kings of the Picts until deposed by Óengus in 728. It is thus that Kenneth was one of several nobles with a claim to the crown of the Picts and Scots.
References
MacAlpin's treason Wikipedia(Text) CC BY-SA