have the leaves got small red brown spots and twigs dies

back if so it is black canker

if the leaves are only brown on

the edge then they could have dryed out as all willows need water or maybe wind damaged

if it is black canker then removes the

leaves and cut out the branch or twiggs they are on and burn

Most likely lack of water, though as you do not say where you are on the planet this is, basically, a guess!