replacing dse fuse box

after noticing my fusebox has the dreaded scorching around the two fan relays ive got to replace it, im waiting for the part to arrive but as of yesterday the fans for the heater have given up, great time to for it to happen with this weather!!!

but before i change over the fuseboxes im wondering what has caused the two sockets to scorch up, is it just a case of the fusebox causing this or should i check anything else before changing the fusebox over??

Any ideas?

Thanks.