This is how daughter prepared cabbage: why does it all go wrong? (4,3)
I believe the answer is:
sods law
'why does it all go wrong?' is the definition.
(everything that can go wrong will)
'this is how daughter prepared cabbage' is the wordplay.
'this is how' becomes 'so' (as in 'like so').
'daughter' becomes 'd' (genealogical abbreviation).
'prepared cabbage' becomes 'slaw' (coleslaw).
'so'+'d'+'slaw'='SODS-LAW'
(Other definitions for sods law that I've seen before include "'The principle that says that anything that can go wrong will go wrong (4,3)'" , "Rule that if anything can go wrong it will" , "Pessimistic theory" , "This is bound to happen" , "Expect the worst!" .)