Foreground emission severely limits the detection of 21 cm emission from the epoch of reionization (EoR) at redshifts z > 6. Radio interferometer observations, through the instrumental transfer function, imprint chromatic signatures of foreground emission onto the measured power spectrum. We analyze these foreground signatures using the delay spectrum technique on an all-sky foreground model. The largest contamination in the EoR HI power spectrum is found to arise from foreground power received far away from the primary field of view. Comparing data from recent Murchison Widefield Array (MWA) observations with simulations separated into diffuse and compact components, we find diffuse emission near the horizon is a significant contributing factor, even on wide antenna spacings. Compact objects dominate the foreground contamination in the primary field of view. The resultant of these two mechanisms is a characteristic Â³pitchforkÂ² signature in Fourier space. Based on these results, we propose a foreground mitigation technique of selectively downÂweighting baselines based on length, direction, and time, which will remove a large fraction of foreground contamination in reionization HI power spectrum analysis.