The video on CNN and MSNBC is very poor quality. I saw much better video on ABC this morning but I couldn't view it on their website because it kept trying to get me to sign up for some Real Video account. I couldn't watch the video on CBS's website because they were having server problems. I did see some clear pictures in this Fox News article. It appears the right wing broke off first, then the left. The wing roots were trailing flame.

Several possibilities about a cause come to mind. Perhaps the most likely is that they clipped a couple trees during their drop run and overstressed the wing attachments. It's also possible (if a little less likely) that they overstressed the attachments by pulling up too abruptly or due to the heat causing very severe up or down drafts. Another possibility is that the wing tanks were on fire due to engine problems (I lost a friend in the early 1990s when that happened to his C-130). It's also possible the wing attach points were cracked or corroded due to age or poor maintenance. That plane was almost certainly military surplus, so it probably had years of hard usage behind it. C-130s are rugged planes but like all machines, they have their limits.