Performance optimization in the large format metal additive manufacturing (MAM) of ultrahigh strength steels (UHSS) is required to take advantage of increasingly complex mechanical component designs. Previous process-microstructure-property (PMP) relationship development led to crack-free single pass wall builds. However, hot cracking was found in solute-rich, interdendritic regions of the fusion zone and weld metal heat-affected zone (HAZ) in thicker section multipass builds. Continuing process optimization required identification of the cracking mechanisms. This work analyzed the cracking mechanisms through metallurgical characterization and computational thermodynamics calculations. Solidification and potential weld metal liquation cracking were related to the non-equilibrium solidification conditions, causing non-equilibrium partitioning of solute elements to solidification grain boundary (SGB) and solidification subgrain boundary (SSGB) regions during DED-Arc fabrication. Solidification cracking was caused by non-equilibrium partitioning of carbon, changing the solidification mode from ferritic to austenitic and expanding the solidification temperature range (STR). Expansion of the STR was compounded by partitioning of impurities, phosphorus and sulfur, during austenitic solidification, forming persistent, low solidification temperature liquid films. Tensile stresses induced by solidification shrinkage and thermal contraction during DED-Arc fabrication restrained the solidifying material, resulting in cracks at SGB and SSGB regions. Weld metal liquation cracking conditions were created by reheating of low melting temperature, solute-rich constituents at SGBs and SSGBs within HAZs of subsequently deposited DED-Arc beads. Formation of a mixed martensite and austenite microstructure was related to solute partitioning in the non-equilibrium solidification conditions of DED-Arc. Reheating during fabrication and the final stress relief heat treatment affected the solid-state transformations.