Looks like someone covered the face-hole in their raincoats with a plastic sheet which was causing them to suffocate to death; the Marisa somehow got it off and is trying to get it off the Reimu but is failing and basically ripping of chunks of flesh instead.
This is all conjecture; my Japanese is too shitty to actually read it
After reading this a bit closely, they weren't suffocating at all, though that's a good idea.
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Yukkuri-shop "Yun-Yeah" Notice:
There are accidents caused by the Yukkuri Raincoat, caused by fear. There are instances where the yukkuris that use said products are hurt because of the resulting fear. There will be no charges for replacing the older version with the upgraded version with transparent hoods.
"The yukkuri without mister accessories die easy!!" "Da-ze!"
I'm just surprised they managed to pull off the visor, seeing as how both yukkuris are pretty much wrapped up in raincoats and they'd be hard pressed to open up the plastic without someone helping them.
The Marisa might have done so by pushing with the tongue, puffing up, scraping against a hard surface or the Reimu while trying to bite, which might explain the damaged eye.
What's bugging me though is, where's the owner(s) in this hypothetical situation?
The accessory is at greater risk of loss or damage and then we're back at yukkuri-on-yukkuri violence.
The accessory has to be removed from the yukkuri. Either the yukkuri itself isn't going to react to this well (especially when you alter it with Velcro or adhesives). Or, other yukkuri will still see the yukkuri as not having an accessory and mayhem ensues.
Most of the stores I have read show that yukkuri that show this kind of behavior will not attack if its a bother to do so. Pet yukkuri should also not attack, but this is an omake, so its more to please the yukkuri haters then anything.