The Lemon Fool

I have well established euonymous
looks like the one above...'japonicus'
its been in a very large 'pot'...9council waste glass bin)
for many years
and has done really well

this year many of the leaves are missing/falling off

is this likely to be 'pot bound'...or some disease?

i would like to keep it if I can cure the problem

any thoughts prerably based on experience as opposed to guesses
(I have read various web sites each of which suggests a different issue)