I spent ages thinking it was Didplis but what I was recently sold as Didplis has much shorter/smaller leaves. Obviously my current "Didplis" could be incorrectly named.
EDIT... This is what my Didplis looks like: http://www.plantgeek.net/plantguide_viewer.php?id=83
It's a plant I had several years ago. Trawling back through my information from that time I thought it might be Ludwigia arcuata; however, looking at other pics I'm no longer convinced - the leaves look much narrower on mine.
I have some Didplis Diandra that does the same. Not worked out why yet - possibly because they're only been planted a couple of weeks in a low-light tank. All the other stems have sorted themselves out though.