They are fake transactions designed to clog the network so that real transactions don't go through or just take a really long time to confirm. That's why it's an attack. They want to stop LTC or at least slow it down. LiteCoin has crazy momentum right now which is threatening their profit. These attacks are expensive and cant last.
RE: Litecoin network is under attack! (disabled on polo, btc-e, GDAX etc.)