looks like a 4 or 5 foot bonsai ... ???? ..... i wonder why you call it such ... it surely doesnt look like a plant that is dug up and root pruned annually.. if not more often ....

where are you .. big city ... how long has it been there.. whats the weather been for the last year.. how out of the ordinary was that weather .... whats your local soil ... how about a pic of the whole ....

it would be good to start with an ID ....

all kinds of things can happen to heavily stressed plants ..... but you have to start .. by trying to figure out the stress ... rather than dealing with what may be secondary issue ....

its the old what came first .. the chicken or the egg ...

more info plz... and welcome to the forums ...

ken

Are there holes in the bark? Fleshy looking sap on the trunk? Caterpillar like things on the top? Did you have a drought? Do you know what species it is?

Might be Pruner Blight, as in you have sheared it back too far into the old part for good recovery.

that's what plants look like after a SEVER infestation of saw fly larve

ron