The initiation of toppling is explored for a uniform stack of blocks that rotates slowly about its mid-base. As the stack passes through its vertical position ($ heta$=0), it is in free-fall rotation, and a critical inclination angle ${ heta}_c$ is reached at which the toppling stack "fails" or begins to crack or separate. For tall stacks (high aspect ratios), two modes of failure are hypothesized, for which the dynamic failure analyses are shown to correlate with experimental results. These block failure modes are similar to those observed for tall, toppling masonry structures with weak binding material between their brick or stone blocks.