You mention the admonition in HSG245 concerning the avoidance of bias. However, the admonition needs to be placed in the context of the rest of that document. HSG245 itself then, has to be placed in the context of the framework (HSG65 1997 2nd Ed.) within which is was designed to operate. In Page 6, a key message is "recognising that accidents, ill health and incidents result from failings in management control..." A point reinforced in the guidance in Appendix 5 (p.87) that "All immediate and underlying causes are in one sense a failing to devise and implement an adequate health and safety policy". Having set up the framework within which HSG245 was created, the definitions used for both 'underlying' and 'root' causes, on page 5 of that document, heavily reinforce the direction in which an investigation should travel. Particularly if it is to meet the policy set out in HSG65.
Whilst on page 10, we are admonished to "avoid bias and leaping to conclusions" the paragraphs which precede this admonishment state "Investigations that conclude that operator error was the sole cause are rarely acceptable" and "The root causes of adverse events are almost inevitably management, organisational or planning failures". Whilst some might refer to this whole approach as being tantamount to 'institutional bias', I am aware that we all must seek to identify our own biases and adjust our modus operandi to negate them wherever possible. There was a really good programme on the radio on Sunday, on just this problem http://www.bbc.co.uk/programmes/b09pl66d
Confirmation bias has been much in the news lately with the failure to take into account other versions of reality which, though available, may result in congnitive dissonance. You rightly point out that 'why' is a very loaded word and is singularly unhelpful when answered, as can be attested by parents and teachers up and down the country and quite often truthfully by, 'I don't know why I did that'. And all of that before we even begin the 'cause' debate. :)