Complex sentence: It is clear that Red had forgiven McQueen and warmed up to him, especially since he burst into tears and fled when he thought McQueen left without saying goodbye (which he did not, but was, unfortunately, forced to do later).
Split into simpler sentences: It is clear that Red had forgiven Lightning McQueen and warmed up to him, especially when he burst into tears and fled. This was because he thought that Lightning McQueen left without saying goodbye (which he didn't, but was, unfortunately, forced to do later).