Ranieri, whose side pride themselves on set pieces, mysteriously conceded three times from first-half corners as they suffered their third Premier League defeat of the season – as many as they encountered in the whole of 2015/16.
The manager said: "It is very strange. We are so strong from corners usually.
"After the first goal we switched off everything. We lost our shape, our confidence, our strength. We are a good team when we play together. It is a good lesson for us. It is important to maintain your composure and stay calm.
"I don't know why it happened. It is the second time it has happened in the league. Now I want to speak with my players, to ask them 'Why?'
"You can always lose games but it is important to keep your mentality and shape. It is important you stay and play eleven against eleven, not play alone. To blame the next Champions League would be an excuse."