PAPR in the SC-FDMA system
This can be explained with the help of some knowledge of probability theory. 
Since the output signal is a hybrid signal consists of many(say,Nfft=2048) single carrier signals, which are QAM-modulated by the complex symbols derived from the information bits. Thus leads to(according to the Central Limit Theorem in probability theory) a near Gaussian distributed signal.

As is known to us, Gaussian distribution is not bounded in its range, hence resulting in a high PAPR in OFDM signal. While, in SC_FDMA, things are different. The output signal is essentially a single carrier modulated by the complex symbol, no summation of Nfft signals here. Thus a low PAPR is obtained.

This can also explained intuitively in the time domain. Say, you have a single carrier(in SC-FDMA) modulated by A QAM modulated scheme. The peak of the signal waveform is almost bounded by the carrier amplitude it self. Whilst, in OFDM, where you have a large number of carriers, each with a different frequency. The peak of the output signal is not necessarily bounded by the highest amplitude, since at some time point, all the peaks of the carriers maybe coincide together and thus sum up to form a ultra high peak. While, at some time point, its negative counterpart can be appeared. Although both of the positive ultra high peaks and negative of it are rare in time, it indeed leads to a high PAPR.