I would imagine it was just another random security scare - there have been hundreds over the last few weeks. I work just opposite the entrance to London Bridge tube and there have been quite a few evactuations of the tube, lots of sirens, sniffer dogs etc and then five minutes later they roll up the police tape and its all back to normal. The best thing to do is probably to go inside, draw the curtains and keep away from the windows (curtains would help stop the glass blowing into the room if a bomb went off) and have a cup of tea!
Yes, there is a very high police presence at all the tube stations and it seems like they are all being costantly opened and closed. Certainly where I work in Brixton it seems shut more often then open at the moment. I'd try not to worry and get on your bike if you need to get around.