Header pipe pitted and crack

I went to paint my header pipe, as it’s starting to look rusty and I usually paint them every so often to keep it looking nice, I noticed a lot of very deep pits, deeper than I remember them being mostly around the center part where the Heat guard covers. I haven’t ridden the 300 all summer and it was sitting outside for probably a month under a tarp. Along with the pitting there’s also a large crack going along the curve of the pipe, where did that come from? Was it really under that much stress outside? I’m more curious than anything, the crack doesn’t seem to be letting any exhaust out but I’ll have to double check. Anyone know how this pitting and crack came about? Attachment 25280Attachment 25281Attachment 25282

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Header pipe pitted and crack-imageuploadedbytapatalk1440957836.032182.jpg
 

Header pipe pitted and crack-imageuploadedbytapatalk1440957846.125665.jpg
 

Header pipe pitted and crack-imageuploadedbytapatalk1440957854.293805.jpg