  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.
