Jefferson and Congress do not use the term "natural rights." In the context of the Declaration, the closest approximation to this concept is contained in paragraph 2 with the famous words,
We hold these truths to be self-evident: that all men are created equal, that they are endowed by their Creator with certain unalienable rights, that among these are life, liberty, and the pursuit of happiness . . . .
The Declaration then, of course, states how these central rights are facilitated: by governments. In other words, the only legitimate reason for a government to exist is that of enabling people to live, to be free, and to be happy—and by the term "pursuit" Jefferson and Congress mean that it is the individual's responsibility to accomplish this without any interference from the government (or others).
The things that have been done to the colonists which are contrary to, or violate, this principle of the role of government are then enumerated. In each case, the king himself is the one principally blamed: "He has refused his assent to laws . . . necessary for the public good," "He has forbidden his governors to pass laws of immediate and pressing importance . . ." and so on, and after many other violations cited, we are told that "He has excited domestoc insurrections amongst us" (italics added). All of these actions by the King are seen as inhibiting the unalienable rights enunciated at the start of the Declaration. But in answer to your question, what is the reason George III has committed these violations? we are told his aim is
the direct establishment of an absolute tyranny over these states.
Throughout English history, there had been a continuous tension between the power of the sovereign and that of the people, the latter being represented by the legislative body, Parliament. If the sovereign was perceived as holding too much power—as was the case with Charles I and James II in the 1600s—then government ceased to be valid. The colonists in rebellion against the Crown considered their situation to be another in this line of conflicts between, on the one side, a potentially (or actually) tyrannical power and, on the other, the valid function of government in serving the people. The central grievance of the colonists, as represented by Congress, was therefore the arbitrary power exercised by the king and his administration against them, and all the individual grievances listed are caused by, or contained within, this one. The solution, as Congress saw it, was political separation from Britain.