I just fail to see a reason.
Or do they try to manipulate the overall hashing rate with it?
That would at least make 'a bit' of sense...
The reason I think is this...
>90% of all mining takes place in top 3 popular pools.
So, if you have a nice big GPU farm, setup for SOLO MINING, then DDOS the big three pools. Then for a while, you get almost ALL of the blocks as you have no competition. This is why it is important to have 5 or 10 pools with load spread evenly, it would be very difficult to take out a larger number of pools than just a small number.
That's it. Just down to greed and cheating.