I'd guess that today's poor relative performance is some fund rebalancing their portfolio and selling into the strength of today's market. If you look at TickerSpy, you'll note that many large holders lightened up after the runup while others - mostly newer holders - added or initiated positions.
Portfolio rebalancing by some fund is a plausible explanation. But then again, other biotechs are near their 52-week highs and do well on days like this. The question is why they are not susceptible to the same price pressures due to fund rebalancing. One characteristic of ONXX is that its price can move by more than $1 with just a few thousand shares traded. It seems to be the perfect stock to be manipulated by shorts. Where is the instutional support, given all the positive analyst comments and increasing PTs? Incidentally, just look at the daily and weakly charts--you can draw very clear straight lines connecting the high and low points.