I surmise the glueless patch held long enough for the tube to stretch everywhere but at the at the patch.
Glue joint fails and the tube stretches in that spot to fill the contour of the rim, lengthening the slit in the process.

I haven't had a glueless patch fail inside a tire, but I size my tubes close to tire size.
When I've had them fail was when I was patching a tube for a subsequent flat.
When I blew up the tube oversize to find the leak, the patch wasn't strong enough to constrain the tube from stretching and failed then.