it is vibration from use. i have 3-4 mufflers cracked there and i just run a bead of weld over the crack for a repair. another bad spot is the left support fastener nut sometimes cracks all the way around and ends up not even part of the muffler anymore. i repair those by just welding the support on. those muffler are expensive here so well worth repairing. they are also built alot heavier then the AM mufflers or even the 372 mufflers. the cover i've seen happen too. you will find the missing plastic is actually kinda melted in the slot it would normally slip down in. no idea why, they just do. not all the time and it happens after some serious hours. the muffler was likely ran loose at some point too. that's a high hour saw and way way way out of warranty.