It's not unheard of for damaged buildings to take a while before actually collapsing. As I recall, both of the WTC towers were still standing over an hour after they were struck, but they're not exactly standing now.
Once the structural integrity is compromised, it would really take very little to provide the final straw for it to collapse, and that could pretty much be anything. Tiny earthquake, nearby explosion, sonic boom, attempts to clear wreckage, or indeed, as I've seen suggested on several news outlets by now, possibly foul play. But it's impossible to say at this point, and I doubt any unbiased investigators (non-israeli and non-lebanese, specifically, as in a case like this neither side can really be trusted to not spin it in their favour) are going to have a chance to have a look at it anytime soon.