Myself I think it broke because its 58 years old and I fly the hell out of it doing hard turns (30-60 degree) and constant side slipping at 100mph
I believe that the Luscombe 8E has a manoeuvre speed of 85mph, so a full rudder sideslip at 100mph will take you to about 8% below the ultimate design load on the fin and rudder - WHEN IT WAS NEW. So, even without quite using full rudder, that'd probably do it, yes.
G