I think one of the sock, despite knowing that he has his one and only soul mate, decided to adventure a bit, so, he left to meet up with other single socks which most likely did the same. As soon as the second sock finds the news, it kills itself due to being unable to support so much pain.
Alternatively, if you still have one sock for a really long time, that one sock just lies in wait hoping that the other sock would come back to him one day and engage in a deep relationship with it again.