Hi
Could be one of many things really, you won't know for sure till they plug in and bring up the fault codes
usually something like a missfire will cause it, so could be any thing like a duff plug, a coil pack, or even a split hose on the induction system
it might be worth investing in a generic fault code reader, that will work on most cars and will tell you a code and fault, these are generally under £30 now
you usually find, if you turn the ignition off and on it will clear the limp mode and drive normal and if you drive normal the fault wont come back, most faults occur when you have your foot hard down or under load, like going up steep banks, thats the favorite time for incorrect fueling, which causes misfires, which will cause the engine to go into limp mode, this protects the engine, as running lean, misfiring or pinking can damage the engine
cheers
Joe