Why do women do this?

Alright so i'm playing WoW and my girlfriend bought these sheets that are supposed to cut down on allergies...it's a mattress cover thing and pillow covers that go under the pillow covers.  I don't know if i'm saying that right but that's basically what it is.  So she is yelling at me that i'm not helping her put these on my bed.  So I get up while they are killing stuff so I get XP :p and I try to help with the bed cover....I rip it.  Now she flips out and i'm like how am I supposed to know u cover the bed then zip it up...I thought it looked like it was caught so I tugged it and it ripped.  So she flipped out because the thing was like 100 bucks and blames me and said I should have left it alone.....I get yelled at for not helping and then yelled at for helping.  I can only imagine this get's worse with marriage.