I have been trying to stay off the cat board as I feel I am taking over the dog board sometimes and I don't like that feeling.....but I couldn't resist this one. I hope y'all don't mind.....

I think there is two main reasons for cats to collect objects, toys and socks especially, although other items of underwear are favourites too!

One is the trophy aspect - the same way that outside cats will bring home their prey.
This will occur when a cat has no reason to hunt for food and is well bonded to it's human family.
A cat cannot help but respond to it's hunting inheritance. It is the most basic of instincts, yes.....even more than the sexual urge....
A cat must learn to feed itself to live to the age of sexual maturity and to bring back food shows that it can kill more than it can eat. This makes it a very desirable mate. It is a basic urge that is transfered to toys and sometimes other items and is aimed at showing the cat to be a superior specimen and meant to impress.
The cat that shows this behaviour tends to have had a normal nest period and good filial bonding (bond to mother). (Not always the case, but usually.)

The second is more like the original post - the cat is usually an orphan or an only kit or has had a disrupted infancy, maybe been seperated from the mother at a very young age. The cat chooses strongly scented items of the owner (once learnt the behaviour often moves on to clean items of the sort that smell strongly when first taken off, like socks) that the cat feels a compulsion to collect together. It may be that the cat is going through behaviour learned in the nest ( mother gathering kittens that share a common smell). As the kit was deprived of full learning from the mother for whatever reason, abandoned, orphaned, removed from nest early.....it has not completed the full range of learned behaviour that full term nestlings have.
Some of these collecting cats will grow out of it, but some seem to persist with the behaviour all their lives.

How about giving the cat a basket of old socks? Spread them around the house before you go out and it will keep the cat very happy and active during the time you are away. Rub the old socks in your hands before you let kitty have them.
Another interesting thing to do is to get everyone in the house to choose a sock and put their scent on it. Then see if there is a pattern to which sock is looked for first.