Your question was asked by the She'arim ha'Metzuyanim ba'Halachah, by Rav Shlomo Zalman Braun, who added that based on the Rambam and Rabeinu Yonah's opinions in Pirkei Avos we are obligated to be Dan l'Chaf Zechus a righteous person even when it is very difficult to justify their actions.

The Avodah Berurah answers that they did not suspect each other of having done an Isur, since Reish Lakish maintains that this is permitted. Rather, each one suspected the other of having ruled like Reish Lakish and therefore they called each other a Yasom, as ruling against one's Rebbi is as if one doesn't have a Rebbi, and since a Rebbi is a father figure, without a Rebbi one is viewed as a Yasom.

This explanation is borne out by a similar Sugya which appears in Chulin 111b, in which Rav Huna and Rav Chiya bar Ashi, both students of Rav, acted differently from their Rebbi and accused each other of being a Yasom. Each then defended his actions based on the opinion of other Amora'im.

It should also be noted that one who diverges from his Rebbi's Psak is considered as if he disputed the Shechinah (Sanhedrin 110a), which should entail a severe punishment. However, since Hash-m is described as the "father of Yesomim" (Tehilim 68:6), each of these Amora'im called his friend a Yasom so that Hash-m should spare him and love him like a son despite his arguing with his Rebbi.

Yoel Domb