When we hear the masaib of Ashura every year, in there we are told how Imam offered the zohr prayers while his companions shielded him from the arrows. Then when Imam a.s was doing jang then at the time of asar he stopped for namaz which is also when his Shahadat was. I hear this from every moulana who reads the masaib. So wasn't it like this?

He prayed Salat al Khawf which is salat in the time of fear. This could have been a reason he would separate them.