Question

why workdone is against the torque

Answer 1

because in an uniform electric field the net force on dipole is equals to zero and only torqe acting on the dipole and if we want to find work in that case we have to find force and Displacement . Here bet force equals to zero but torque acts on it and tries to get an angular Displacement so we can find work by using this fact that if τ dθ is the work done by force so same work we have to done in rotating dipole along a Displacement θ.