probably rotten floor joists
yes it normaly happens because of moviment on the floor. The floor needs to have a 20 mm gape between the floor and walls to invite does situation to happen.
Has a wall been removed from underneath at some point? Sounds very likely a loadbearing wall could have been removed to open up space and the floor is inadequately supported if the dipping is roughly central to the floor. If the deflection is near a external wall or corner then it's possible the joists could have rotted away which would cause the floorboards to dip. Best bet is to try to lift the floorboards to check the joists underneath and take it from there.
Sounds like the supporting joists need to be strengthened bad news whole lot up re support and floor back down
I would need more information may some pictures or detailed information. Regards PNZ