Maybe some tax loss selling going on to offset the big gains folks have made elsewhere. Weird that there was 34.2k share buy at .4997 immediately followed by a 99.8k share sell at .4852 when the bid ask spread hasn't been that wide all day.
I think they're matched orders myself. Somebody (or organization) is basically buying and selling to themselves in different accounts in order to capture gains or losses. I've done it myself before between cash and IRA accounts at different brokerages. The account holding the position, and its cost basis change, but the net position stays the same.