spring tab-flutter- fatigue
This sounds a highly likely cause - perhaps leading to loss of control and a spin?
How was recovery from those steep dives effected? Was pitch trim used in an attempt to assist with heavy control forces?
One wonders whether the pitch circuit, rather than empennage, was subjected to loads beyond those it was designed to cope with?
Repeatedly playing Stukas with a Viscount is unlikely to have been beneficial to aircraft fatigue.