Perhaps the one word that can destroy rapport faster than any other during a conversation with a client is the word "why". When you commence a sentence with 'why' when you want to know the reason for their actions, it places unnecessary blame on the client. A better phrase to commence a sentence is "What was the reason for..." It is asking the same question yet much softer and without a direct accusation.
An example; "why was the door unlocked" as opposed to "what was the reason for the door being unlocked". It is less accusatory and provides the client with an open opportunity to explain. In this way you are more likely to get an honest answer because they won't feel threatened.
There is nothing wrong with asking why they think something occurred provided it is not directly related to their own actions. Asking "why do you think that happened" provides an open invitation to provide their thoughts.