This is not an "instantaneous" thing.
GlaDOS is a sentient AI. She has the ability to reason, justify, is completely self aware (i.e., she knows she is an AI that runs the Aperture Science test facility). In sci-fi, this can get dangerous. Long and short, GlaDOS is a rampant, sentient AI.
"The experiment is nearing its conclusion. The Enrichment Center is required to remind you that you will be baked, and then there will be cake." - GlaDOS; Portal 1 - Test Chamber 18.
Doug Rattmann, one of the few survivors in the Portal series, points out that GlaDOS has become dangerous, doubting the effectiveness of the Morality Core that was installed. His fears are confirmed, when she attempts to murder everyone with Neurotoxin.
"You can always ignore your conscience" - Doug Rattmann
The events of Portal 1 lead to Chell destroying several of GlaDOS' cores, starting with her Morality Core.
In the events of Portal 2, Chell and Wheatley gang up on GlaDOS to run a fail-safe procedure in an attempt to remove GlaDOS, and take control of the facility. This voice isn't anything more than the facility PA system reporting the results of the test, which has more than likely not been run before now; hence why GlaDOS is still in charge of the facility.
This "Corruption" is likely a test to check the integrity of the AI, and its "morality". Since GlaDOS is responsible for mass murder, torture, imprisonment, etc., it is fair to say that she is pretty damn "Corrupt".
Though, following the events of Portal 2, it becomes apparent, since she decides to forgive, and even reward Chell by releasing her once GlaDOS retakes control of the facility, that she is not completely corrupt, and therefore she is able to redeem herself.
So, to recap, the test that reports an "80% corruption" is not an instantaneous "spike". She has, over time, built up a corrupt sense of morality, and this is the first time this test has even been carried out, since she had previously killed anyone that might have been capable.