Enforced Secession?

It is a rare event.

The events in 1965 when Singapore left Malaysia have been called expulsion, separation and independence, depending on on whose history book you read.

If you want to stress that it was involuntary, then expulsion looks like the best word.

According to Wikipedia, West Virginia broke away in turn from seceding Virginia during the Civil War, thereby "seceding from secession."

diremption

a forced sundering.

I first came across it in a translation of Hegel. The original German is, Entzweiung - or, split.

From the Latin, it appears to be used transitively - thus its pertinence. The state, or the self, for Hegel, dirempts itself and cleaves a withered portion away.