a) Because we've been watching pub teams hump it up in the air for 7 years and we actually go to matches.
or
b) It's an RSI or repetitive strain injury caused by either too much polishing our cups with brasso (like the Golden gate Bridge once you've finished you have to start again) or by acting out famous cup/title winning goals in our back gardens for our children's benefit.
Get yer facts right and at least get's have some funny jokes on here