I have never had a problem with my rear washer hose uncoupling but I would suggest first checking to see if there are any kinks or restrictions in the hose that leads to the rear washer nozzle, if there is this could cause excessive back pressure and could cause the hose to blow off the connection barb. I would also check how tight the connection is at the coupler, if it is rather loose this would also cause the hose to blow off when the washer pump is engaged, also check the hoses where they connect to the barb, they have a tendency to stretch over time or become stiff of brittle and loose their elasticity and therefore their ability to hold tight. Good luck.

My .02 cents
Bill