It means they have huge liabilities on Murray so they don't want to lay him anymore.
If Murray wins they only have to refund your stake rather than pay you out (if you back someone else).
It also means that Murray's price is too short.
Paddy Power, BetVictor, Stan James and the Magic sign are all refunding losing bets if Murray wins.
Shirley there is money to made here (I know **** all about tennis mind)
Dutch:
Djokovic 5/4
Nadal 4/1
Federer 9/1
???