Busy.org works as well. Also Steemit Chat works. So I suppose the target is more Steemit.com. And some of the Apps, but because they may use some API from Steemit UI instead of an API of the blockchain, not sure. Lets see how this will evolve.
Reasons can be any you mentioned. Cryptospace is a very uncontrolled space without many rules. It is kinda the wild west with the cowboys. Any nasty practice that in the 'normal' world can cause serious reputation damage or even court cases, seems to be normal behaviour in cryptospace. I think the anonymity is one of the key drivers for such practises.
DDOS attacks are a common thing, sadly enough, on the internet.
So far it is unclear what happened. Sometimes SteemIt dot com works. Sometimes eSteem works.
Strange...