"There is a Charm called Obliviation."

Harry froze in place. "A spell that erases memories?"

McGonagall nodded. "But not all the effects of the experience, if you see what I'm saying, Mr. Potter."

-

"Obliviation cannot be detected by any known means"

-

"Miss Granger has been obsessing over Mr. Malfoy since the day that Severus... yelled at her. She has been thinking of how Mr. Malfoy might be in league with Professor Snape, how he might be planning to harm her and harm Harry - imagining it for hours every day - it would be impossible to create false memories for so much time."

"The appearance of insanity..." Severus murmured softly, as though he were speaking to himself. "Could it be natural? No, it is too disastrous to be pure accident; too convenient for someone, I have no doubt. A Muggle drug, perhaps? But that would not be enough - Miss Granger's madness would have to be guided-"

"Ah!" Harry said suddenly. "I get it now. The first False Memory Charm was cast on Hermione after Professor Snape yelled at her, and showed, say, Draco and Professor Snape plotting to kill her. Then last night that False Memory was removed by Obliviation, leaving behind the memories of her obsessing about Draco for no apparent reason, at the same time she and Draco were given false memories of the duel."

In chapter 75 Snape gives Hermione a rather thorough dressing-down in front of the school. Did the Groundhog Day attack in chapter 77 happen on the same day? I have been assuming that Snape & Harry were broadly correct, and the Groundhog Day attack was how that madness-guiding trick was done.

My working-theory is that the entire purpose of the attack was to produce a directed trauma, and an obsession, without a readily detectable cause.

Xachariah and Drethelin, if I've read you right you both believe that Hat and Cloak's purpose was to put some idea or belief into Hermione's head; to convince her of something. I think that there is something rather deeper and more subtly manipulative going on here.

[Edited for formatting.]