SN 2021aefx is a normal Type Ia supernova (SN) showing excess emission and redward color evolution over the first ~2days. We present analyses of this SN using our high-cadence KMTNet multiband photometry, spectroscopy, and publicly available data, including first measurements of its explosion epoch (MJD59529.32+/-0.16) and onset of power-law rise (t_PL_=MJD59529.85+/-0.55; often called first light) associated with the main ejecta 56Ni distribution. The first KMTNet detection of SN 2021aefx precedes tPL by ~0.5hr, indicating presence of additional power sources. Our peak-spectrum confirms its intermediate Type Ia subclassification between core-normal and broad-Line, and we estimate an ejecta mass of ~1.34M_{sun}_. The spectral evolution identifies material reaching >40000km/s (fastest ever observed in Type Ia SNe) and at least two split-velocity ejecta components expanding homologously: (1) a normal-velocity (~12400km/s) component consistent with typical photospheric evolution of near-Chandrasekhar-mass ejecta; and (2) a high-velocity (~23500km/s) secondary component visible during the first ~3.6 days post-explosion, which locates the component within the outer <16% of the ejecta mass. Asymmetric subsonic explosion processes producing a nonspherical secondary photosphere provide an explanation for the simultaneous appearance of the two components, and may also explain the excess emission via a slight 56Ni enrichment in the outer ~0.5% of the ejecta mass. Our 300 days post-peak nebular-phase spectrum advances constraints against nondegenerate companions and further supports a near-Chandrasekhar-mass explosion origin. Off-center ignited delayed- detonations are likely responsible for the observed features of SN2021aefx in some normal Type Ia SNe.