[Paul Riley]

The fact you were only in 40m is probably the reason you sunk from the DC run.You had less space between you and the DCs making near misses cause more damage.
When being DCd you want to be in as deep water as possibe,i'd say at least 100m,which would give you more time to evade the DC.
There may be other reasons,but I reckon this is the main reason.
And,using TC while being Dcd is bloody risky mate,I never do that.I give myself about an hour of real time from when the DC attacks began,stop engines,check for sounds,then carefully make my ascent,check through PS at near and far range,then blow tanks at Flank speed,as even surfacing you are vulnerable from attack.