Why did the Van Graffs try to kill me?

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.

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.

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