Why I'm surprised to get backing (4,3)
I believe the answer is:
what for
'why' is the definition.
(for what reason)
'i'm surprised to get backing' is the wordplay.
'i'm surprised' becomes 'what' ('what' can be an expression of surprise).
'to get' becomes 'for' (as in 'to go shopping for presents').
'backing' says to put letters next to each other.
'what'+'for'='WHAT FOR'
(Other definitions for what for that I've seen before include "reprimand?" , "Beating" , "Punishment that may be given" , "it's a question of purpose" .)