Ion energy distribution functions measured for high power impulse magnetron sputtering show features, such as a broad peak at several 10 eV with an extended tail, as well as asymmetry with respect to $\mathbf{E}\times \mathbf{B}$ , where $\mathbf{E}$ and $\mathbf{B}$ are the local electric and magnetic field vectors, respectively. Here it is proposed that those features are due to the formation of a potential hump of several 10 V in each of the traveling ionization zones. Potential hump formation is associated with a negative-positive-negative space charge that naturally forms in ionization zones driven by energetic drifting electrons.