Ive got a renault megane scenic 1.9dci 53 plate with masive engine failier due to cam belt move ment. ?

The list is endless, from needing a new head, cam shaft , cam belt , water pump, turbo, ect ect to the total of � 2,462. My question is the garage said that the top cam belt nut on the pulley was only hand tight, could this mean that the vibration in the pulley made that cam belt move off the pulley and start to rub on the casing put added pressure on the bottom pulley which eventually snaped the bottom pulley the cam belt it self was not broke except fraying on one side, could this have been a long going problem before the final demise of my engine . Many thanks