Different makes and models have different reasons. It can also vary by which version of the software is installed. Should you wish to find if this is avoidable for your freeview recorder you will need to find out the make, the model and the software version. The software version can be located via the menu button on the remote.
Some faults will delete all the recordings and some will delete only some of them. Some will be OK when switched off but will have a reduced recording when switched back on. Others delete while you are using them. Some makes will freeze more often than others and switching off at the mains during this state can sometimes, but not usually, corrupt the contents of the disk.
I doubt that any one person can answer your question for all makes, models, software and circumstances.
A reason that goes across all makes and models is that someone else is deleting them. You need to make sure that they do not delete the recorded programmes.
NS, July 2012
