I would venture an opinion like this.
Neutral connection at receptacle 'loose', and over time arcing from bad connection further deteriorated the condition. Heat transfer thru the recept transferred to the cord cap, causing further deterioration. Start-up cycles of the freezer added to the issue.
As to the cb not tripping...it's only monitoring the current on the 'hot' leg, and IMHO it's very doubtful that a fault to ground occurred in this situation.
