This is of course not a tradition that is specific to the house of Juliet. It is believed by some that writing your name and that of your loved one on a lock and then attaching it to a bridge, a fence in a picturesque location, or in this case a gate in the grounds of Juliet's house will guarantee everlasting love.
Love locks have been attached to a significant number of landmarks around the world, including here in Australia. This has caused some structural problems; with older bridges the weight alone can be an issue, and with metal constructions having locks attached can hasten the process of corrosion. A lot of places have therefore started to remove them.
However this is slightly different; this particular ornamental gate would have been put there specifically for the purpose of holding the locks. I really have no idea much space is left there and I suspect that periodically (much as with the letters that are famously inserted into the wall of the building), they are cleared off by the house's staff.
Just don't tell the couples who have left one of their locks there!