Why did the Van Graffs try to kill me?

Solution 1:

The Van Graffs and their thugs are set to become hostile as soon as you complete the quest in a negative way for them.

If you need an in-game explanation, they might have an informer in the NCR army.

Solution 2:

As of patch 1.2 taking the non-violent path to "Heartache by the Number" makes the Van Graffs hostile to you even if you weren't caught during the mission.

Be careful because killing Gloria will result in Rose of Sharon Cassidy wanting to complete the quest the "violent" way. On the upside it is possible to get the Hand of Vengeance perk if you have changed your mind.

Solution 3:

Is it possible you were dressed as the NCR or some other faction?

Solution 4:

Yes, they are automatically set to become hostile after the patch. Just kill Gloria first and Cass will want to kill the Van Graffs. Choose to loot them. You can then wipe them out and steal all their stuff without penalties. Plus you get both of Cass' perks if you do peaceful first. That may be a bug, but it is proven you keep both. Not sure what will happen if you go back to the outpost. About to check it out, but by the looks of it, nothing, since it didn't give me any NCR infamy.

Solution 5:

The Van Graffs get hostile after ratting them out to the NCR, and that's fine with me, it's pretty logical really.

The thing is that the quest to peacefully "take down" the Van Graffs and the Crimson Caravan (Birds of a Feather) fails as you enter the part of Freeside that has the Kings, the Wrangler, the entrance to The Strip and the Van Graffs (kind of hard to avoid for the rest of the game), even if you don't shoot back since the Van Graff thugs will get attacked by every other Freeside NPC after they attack you.

This means that you can't really finish the game with the peaceful Cass quest, since it will fail forever if you even enter the above mentioned part of Freeside. Basically they patched the game to automatically break one quest, or at least that's how it looks to me, taking all things into account. Am I wrong?