Historical Linguistics and Language Families 1, Exercise 3: causes

going to -> gonna        economy (least effort), the source's own example
loss of /r/ in RP        social prestige, the source's own example
terrific -> "very good"  expressiveness is the closest fit: overused strong
                         language loses intensity, so a word meaning
                         "terrifying" was weakened. The source classes the
                         case as amelioration but does not state a cause for
                         it, so this match is my judgment, not the page's.