We analyze the directed flow of protons and pions in high-energy heavy-ion collisions in the incident energy range from $sqrt{s_{{scriptscriptstyle NN}}}=7.7$ to 27 GeV within a microscopic transport model. Standard hadronic transport approaches do not describe the collapse of directed flow below $sqrt{s_{{scriptscriptstyle NN}}}simeq 20$ GeV. By contrast, a model which simulates effects of a softening of the equation of state, well describes the behavior of directed flow data recently obtained by the STAR Collaboration~cite{STARv1}. We give a detailed analysis of how directed flow is generated. Particularly, we found that softening of effective equation of state at the overlapping region of two nuclei, i.e. the reaction stages where the system reaches high baryon density state, is needed to explain the observed collapse of proton directed flow within a hadronic transport approach.
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