So here goes the first theory
In the last one piece chapter 733 we saw Block D being sabotaged or some type of accident occurring leading to a quicker ending to the qualifier. Who did this? Well..this is what this theory is about ^_^
So firstly, I will consider Sabo as being the guy who unleashed this Haki or whatever it was on the contestants. I know Sabo has been a major plot reveal in the previous weeks but him sabotaging the tournament this way makes no sense at all. Why would he go to all the hassle of wearing Luffy's disguise to try get the mera mera no mi if he was going to sabotage the whole tournament? Besides, he is seen in the right bottom panel of the last page saying the words "I heard rumors about it". Sabo is now wearing Luffy's helmet and disguise and considering the words he stated, this leaves little doubt as to his being innocent with regards to what happened down in the coliseum.
Before trying to think of other people, we should consider what actually could have happened. Is it haki or something else? Starting from the initial idea that this is actually haki, this "haki" has brought down 280mln bounty Cavendish, Rebecca and all the rest of the contestants who are all pretty much quite good in terms of battle if they even considered going in to try their all in the tournament. We're not talking about some random guys who can easily be brought down by any Haki. With this said, if it is Conqueror's Haki...then it can only mean people with an immensely strong Haki.
Blackbeard can be one such guy but I disagree for a number of reasons. Firstly we have yet to see whether Blackbeard has conqueror's haki at all (and in my opinion he doesn't - he just obtains whatever by stealing/bribery rather than have it in him, and you cannot train conqueror's haki unless you have it in you). In addition, why would Blackbeard talk to Burgess through Den Den Mushi's rather than meeting with him if he were truly there?
Shanks - well he truly is a Haki monster. And if he were the one to sabotage the tournament this way, that would put 4 yonkou's directly and indirectly involved in the same arc! However, what would be the point of Shanks sabotaging such a tournament? I guess Shanks would benefit of Doflamingo's and Kaidou's defeat but if a yonkou had to directly come into a full war against a subordinate of a yonkou, there would be little or no space for Luffy to even do anything, rather watch as Shanks tears Doflamingo apart. And besides, Shanks ain't really the type to wage war himself, rather tries to avoid battle at all costs.
And this leads to my own theory - Monkey D. Dragon. After the introduction to this arc of the revolutionaries with Sabo and Koala, I believe it's time for Dragon himself to make a move. What is the point of the revolutionaries other than to bring peace to countries under tyranny? And Dressrosa is one such place. If the sabotager did use Haki, this could still mean Dragon as we do not know the limits of Dragon's Haki and I believe it's pretty high up the scale considering his son's Conqueror's haki. If it is not Haki, Dragon could simply have used his wind power to knock out all the opponents without anyone noticing anything. It is a fast and efficient weapon Dragon has. Sabo may have been working under him, but this does not mean he has been on many missions with him directly involved and thus may not have seen his wind power in action yet thus his statement could still stand.
Considering Sabo was surprised at the power, I do not particularly believe it was Conqueror's Haki. I believe that if it were Haki, Sabo should not have been that surprised. The wind power Dragon yields can however be new enough for Sabo to literally see displayed in this tournament. It is about time for the revolutionaries to step up and try do something in this world as they have been hidden away for quite some time. What better place to make a move forward with their leader than Dressrosa?
So in a few words, my theory is that Dragon sabotaged this tournament in one way or another (even if Rebecca will stand up or someone else - Dragon may have simply wanted to have control on the tournament by deciding who wins the Block). What are your opinions on the matter and are there any other possible characters which I missed out?