ליחכה נירו וסכסכה אבניו –
It scorched his plowed field and charred his stones
According to רש"י the meaning of משא"כ בבור is that damages to קרקע (such as דש בנירו or ליחכה נירו וסכסכה אבניו) are inapplicable by בור. However,
תוספות challenges and asks that the גמרא could have offered more common examples that are שייך by אש and not by בור, such as burning a house.
תוספות explains משא"כ בבור to mean that בור is פטור when something unusual happens by a בור; if it killed 'בפחות מי.
According to the רשב"ם the גמרא mentions כגון פפא בר אבא to explain why they are חייב; for if regular people break a bench while sitting on it they are פטור since it is considered מתה מחמת מלאכה. However ר' עזריאל and the ר"ת disagree with this ruling and maintain that פפא בר אבא is mentioned to explain that the others are פטור when he leaned on them, since they could not stand up.