My swimming lane was hijacked!

Did my second tri today (super sprint) and had a hoot (my first tri was in 2006).  Ended up placing second in my age group which shocked me, mountain bike with knobby tires and all! 

The only thing I was not happy with was during the swim portion (it was in a pool) everyone had 15 second staggered starts and we were going up on side of the lane then up the other side, then crossing over to the next lane doing the same thing then out (200 meter swim).  So here I was going along my merry way when all of a sudden a guy jumps into my lane from the deck of the pool around the middle of the length and starts swimming RIGHT in front of me......sidestroke or dog paddle.  I'm not a fast swimmer but he was definately slower than I am.    My first thought was couldn't you have waited two more seconds until I went by you before jumping in?  My second thought was, who is this guy and how come he gets to jump into the middle of a lane from the side of the pool and just start swimming?  Anyone know why/how this might be allowed????