Now this one here is from USA and a SLD killed a guy so an investigation ensued.
http://www.sacbee.com/101/story/170150.html
What is apparent is there's some degree of VTA if you know what to look for. In my experience of SLD the branches have been too long, poor taper, poor angle of attachment/branch too horizontal. What also seems apparent is that individual trees have a history of it.
Here is another, this time an oak.
http://72.14.253.104/search?q=cache:...&ct=clnk&cd=13
Ironically the arborist here has a differing version of what causes it, lack of wind he reckons allows moisture to build up in the limb and foliage, less transpiration I suppose. But either way it appears it's a build up of moisture making the limb heavy and the limb mass itself along with how well it's attached. Below is the pic from the article I leached plus a copy of the article in text in case the link goes bad
(article date was Monday - July 24, 2006).
