Keep losing my custom recovery going to 5.1.1

This has happened twice now with 5.1.1 builds. I'm on a vs980 w/ 2.7 TWRP, and the last few times i've flashed to 5.1.1, i've lost my recovery. Everytime i try to boot to it, i'll get a "secure booting error" message. The only fix (i've found) has been literally going back to stock, then going through the process over again. Does anyone know how/why this keeps happening?
Process was from stock (27a > rooted > twrp 2.7 > 39a bumped bootstack > 5.1.1 slimrom
http://forum.xda-developers.com/show....php?t=2608271

I know the thread says bumped, but could this be some odd "loki vs bumped" error for vs980?