After Lee's surrender, the south needed someone to blame and Longstreet fit the bill perfectly. He had disagreed w/Lee about multiples items at Gettysburg, though we don't know the exact sort of communication that was to occur in that slice of time and if Lee had sought opinions. Also, Longstreet's post war actions set him up perfectly.
Longstreet had lost 3 children during the war and that had to mentally work on him, contributing to a blunt persona. Personally, Longstreet is one of my favorite generals of the Civil War. I think he was a soldier's soldier and though he would follow orders, in his position to Lee, he felt compelled to offer his opinions.