It's
change. If you have received (say)
BTC1000 in one transaction and you spend
BTC1 of that, you can't just spend part of it, you have to spend it all at once, so you send
BTC1 to the recipient and send the remaining
BTC999 to yourself (and most clients will create a new address just for this purpose). Then when you want to send another
BTC1 to someone, again you send the remaining
BTC998 to yourself (again, at a new address). So you end up with a chain of transactions each taking a small amount out and sending the remainder to a new address. This is perfectly normal and happens every time someone receives a large chunk of bitcoins then spends it in small chunks. There's nothing suspicious about it.
