A siphon is beingcreated in your drainage tubes. There is nothing which stops water from coming out of your machine, the only thing that prevents this is that the discharge hose outlet is higher than the water level in the tub.
In your case either the discharge hose is lower, or you have somehow sealed off where the discharge hose enters the discharge pipe (a lot of times this happens because someone taped the hose in to keep it from flipping out)
Anyway, fix one of these situations and your problem will go away.
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