According to the Lynas CEO the market reacted the opposite of what she had expected, page-7

it is 10:26 in US ASC closed at AUD 5.85. Times .78 exchange rate is USD 4.56 CY is 4.54 DY is 4.45. all that counts is close just thought i would give a interim update. There is a very good argument that LYC.ax has already seen the Impact of US and other exchanges. Mellon that controls DY and the multiple small brokers that control CF. by SEC rules have to stay within certain prices of LYC.AX for a single day this range is large but over time the average has to narrow. The spike in volume at end of day could have been these market makers dumping shares from their holdings in preparation of large sell activity on their exchanges. You never know what is going on totally, but to understand how different things work is important.