I found following error in their eventviewer:
Faulting application OUTLOOK.EXE, version 12.0.6514.5000, time stamp 0x4a89dc70, faulting module ntdll.dll, version 6.0.6002.18005, time stamp 0x49e03821, exception code 0xc0000374, fault offset 0x000afaf8, process id 0x14f4, application start time 0x01ca830f85dbedc2.

Maybe a plugin was causing this issues. Running outlook in safe mode disables all the plugins.
You can start outlook in safe mode by right clicking on the shortcut, select properties and add following parameter at the end of the Target command /safe
so your target should be something like this: "C:\Program Files\Microsoft Office\Office12\OUTLOOK.EXE" /recycle /safe

After letting one of the users who experienced these problems start outlook in safe mode, outlook didn't crash anymore. So, one of the plugins was causing the problems.

After comparing the plugins, the "problem" users were using. I noticed that the only similar (and not standard) plugin was "PDFMOUTLOOK", a plugin from adobe. This must be the plugin causing the crashes ! Go in outlook to tools > Trust Center > Add-ins to view all installed plugins.

I let the users remove the plugin ! You can remove a plugin by going to the Outlook tools menu > Trust Center > Add-ins > Manage: COM add-ins > Go. In this window you can select a plugin and click remove.

Regards,
Valérie

Note: If you only have one user with this problem you can start outlook in safe mode and enable plugins one by one, and see on which one it crashes.