Why Does Krum Catch The Snitch?

[cynthiaanncoe at home.com]

No: HPFGUIDX 26576

I tried searching to find the answer to this question, and I came up 
empty, although I am sure it has been discussed before.

In GoF, Bulgaria is trailing badly when Krum catches the snitch, 
which gives the victory to Ireland.  F and G win their bet.

But why does Krum catch the snitch when he knows, or ought to know, 
that Bulgaria would lose if he did it?  We have seen Harry hold other 
seekers off of the snitch when he knows his team doesn't have the 
points to win.  JKR must have something in mind here, but I must 
admit that I have completely missed it.  

Can someone straighten me out, please?

Cindy (a little embarrassed that she can't figure this out for 
herself)