Been there.
Basically warm(er) air condensing on the cold bitumen sheeting behind it then dripping down on to the boards. You only get them popping on one side as the weight of the roof tiles is basically pressing down on the boards at the joints (each board is butt jointed to its neighbour behind the joists)
Very little you can do once they are bent aside from pushing them back up and screwing short pieces of timber strapping on to the side of the Joists. Try taping over any curved horizontal joints to avoid air getting in that way. Its a b1tch of job to get in at where the roof slopes @ the Eaves.
You need to stop the flow of warm / moisture laden air into the loft and increase the ventilation of colder “outside” air into the space. Increasing the number of Soffit vents is a cheap and easy fix. Also double insulate the loft hatch and plug up all sources of heat escape with loft insulation @ pipe cut outs etc. You can buy “Loft Caps” to fit over halogen light fittings if you have any and they in turn can be insulated over. Lastly don’t dry wet clothes (or drape them over warm radiators) in the house.