Indecit W1 DL126-why is their flooding from it and brown sewage water?

Why is there brown sewage water coming through the front door of the washing machine, when the machine wasn't even in use.Tenant came back to flat to find floor was flooded. Its working fine now and has done so for a number of weeks until.... We have run a few cycles and ok now. Just waiting for the next time it happens. Could this be to do with the 'hook end support' not being fitted on in the first place? Can anyone help.