Mr Wooby,
Before the flight, the flight deck receive a weather report telling them of any thunderstorms, wind speeds etc etc on route to the destination.
They can see in advance if an area appears to look as if there may be any turbulence during the flight.
The cabin crew are then briefed by the pilots before we board that, for example, 3 hours into the flight, there may be bumps.
The seatbelt sign is then turned on by the flightdeck 3 hours into the flight as a precaution.
Most of the time there are no bumps at all; the weather has changed whilst we were in the air, conditions are fine. However, if the pilots didn't put the signs on despite the fact that they knew that turbulence may occur, we could then be liable for not telling people to secure themselves.
We do not serve tea/coffee/hot liquids during times of turbulence, whether it appears to be bumpy or not... all it takes is one sudden bump and pax have got burns all over them.
If no tea coffee was served 5 times then unfortunately it was all just a coincidence.
(but you can always ask for one once the sign has gone off)
Having said all that.......
our proceedures changed recently, and it now up to the captain's discretion as to whether we are allowed to serve hot liquids when the seat belt sign is on.