It's a difficult one that, identifying flags in black and white pitures is a nightmare. The "why" could possibly be one of the many occasions the British Open Golf Tournament has been held at the Royal Liverpool Golf Club. You can also see a Stars n Stripes just behind the tree on the opposite side of the banner from the Union Flag. The Open has been held at Royal Liverpool eleven times now, the firsat in 1897 to the latest in 2006. Other years have been 1902,07,13,24,30,36,47,56 and 67. There isn't really anything in the picture to date it wit though.
I thought i may have been a coronation or some other celebration but you don't really fly other countries flags on those occasions.
Here's to 2014 when the Open returns once again!
Wish that was in colour; I'd love to know what tricolor is flying & why.