There are most likely individuals or groups who hold (and hoard?) large sums of Bitcoin because of various reasons. I believe these people are the early adopters or miners of Bitcoin. Basically, we pretty much do not know anything about these people. Will they be a risk in the future?
Large Bitcoin holdings are static assets. The price only climbs if there are active trades. It would be in the interest of large holders to promote the use, trade and circulation of the currency.
If a large holder were to dump all their coins on the market all at once would cause the price to drop, but that would not be in their own interest, as they would lose their investment. Then again, they can only get a return on their investment if they sell it. It would be sensible that if a large investor wanted to liquidate their holdings, they would trickle them out slowly to keep the price high.
Also, the total amount of money wrapped up in Bitcoins and the risk involved is not enough to attract really serious investors. There are some large investors, such as the Winklevii, but they are attracting attention because they are the exception, not the rule.
Basically, we pretty much do not know anything about these people. Will they be a risk in the future?
Basically, we pretty much do not know anything about these people so it is very hard to say. It also depends on what you mean by a threat..
A threat from their ideals? Can it be much worse than the monetary powers who are already present?
A market threat? Even "large" holders will struggle to influence the market, every large spike gives a redistribution of wealth. Those people who were holding a lot of coins generally sell when they see an opportunity. People wont sacrifice their quality of life for money.
I've not investigated a lot but there is an attack to the bitcoin network called "selfish miners". It talks about people mining and keeping their bitcoins without "using" them. Maybe it could work also in this scenario.