I don't think this happened to Flickr before, but it has happen to other companies.. 

But flickr has become more popular and the event around tiananmen square happened on June 4th, so every year the authority are more agressive around this time...

For instance instance you cannot access wikipedia from China and most wiki-based sites are not accessible either..

there is a work around Flickr using a firefox extension https://addons.mozilla.org/en-US/firefox/addon/4286