I had this problem too and emptying the caches and resetting safari was not helping. Maybe there is multiple causes for this, but in my case it was dashboard. I found out it was caused by one of the widgets on my dashboard, likely one of the animated GIF's. I kept deleting them one by one until eventually I got the right one and the problem stopped. However, you only have a second to delete them each as it will slide over to safari very quickly. You need to hold down the option button while the cursor is hovering over a widget, and quickly click on X. The screen will likely slide over to safari. Just slide back, and repeat until you get the right one and the problem stops!!
I tried this and yes it did work. I deleted every widget and tested it, my culprit was the currency & metric convertor widget and the Twitter page stopped popping up once I deleted the widget.
Once I had all my widgets gone and then rebooted; everything was fine but my big mistake was trying to add widgets back after udating OS and rebooting thinking the problem was solved. The problem came back so I deleted all the widgets & rebooted again but even with an empty dashboard/no widgets the twitter page still opens up when I go to my empty dashboard.
So don't try to add widgets back to dashboard otherwise your in trouble. Problem not really solved.
Now I'm really bothered by this, it's like I have a virus on my windows computer that I can't get rid of.
that's too bad. this is a real annoying problem. Maybe try resetting the dashboard completely? I haven't done that myself but it appears it can be done quite easily and may help. Search the forums for how to reset it. I found a post that said to do this to reset dashboard:
try removing these files from /Home/Library/Preferences/: