Learn English – What should follow the word crazy, About, or Over, or both

You have identified the appropriate sense of over, #19; but the key to this use lies in the example there, not the definition: ‘to quarrel over a matter.’ Over is used in this sense specifically when the complement designates the cause of disagreement or strife.

Wars are fought over religion, over political theories, over national identity—but mostly they are fought over money and land.

The sisters wrangled bitterly over who would get their mother's silver.

There was considerable debate in the summer of 1945 over the use of the new atomic weapon.

The extension of over complementation from verbs signifying conflict to similarly impassioned one-sided actions is fairly recent; it seems to be a particular favourite of journalists:

White Marine Beaten Severely by Black Men for Revenge over Ferguson Shooting —headline, Newsiosity

Mum's fight for justice over son's forestry death —headline, TVNZ

Rotherham: police need outside force to 'deliver justice' over child abuse, says Nick Clegg —headline, The Guardian

I cannot recall on being used in contexts like these.

Both constructions are possible, and yes, they are mostly interchangeable. In your example, combined with "how something happened", there is, I'd say, no difference. However, if we are pairing this with nouns, there is a slight difference. For example, there is a classic phrase:

There is no use crying over spilt milk.

You cry over something when it's an object that got destroyed or that you lost.

You cry about something when it's a general source of unpleasantness or an abstract situation. E. g., you can cry about someone's insensitivity, about someone's unpleasant words, or similar things.

When someone cries about their haircut, it's not really about a destroyed object, it's a situation where a hairdresser messed up someone's haircut. However, keep in mind that this distinction between an object and a situation is not too strict; the article I linked to also says this person "cried over her hair", because the whole matter is slightly ambiguous with regard to whether it's the object we are crying over/about, or the situation in which the object was spoiled/destroyed. Because of this general uncertainty, usage of "over" and "about" will often overlap; this is why I'm saying they are mostly interchangeable.

When "cry" gets combined with a clause, it is entirely possible to view a whole situation as an "object" that gives you cause for grieving, and then you can "cry over how much you want something", "cry over how much you love something", etc.

When you are asking someone a question, the default one would be,

What are you crying about?

because this might be either an object or an abstract situation.