Fact: A dead O2 sensor will show up as lean to the computer.
Theory: Nailing it to get on the freeway, the computer added fuel because of the increased load. When the O2 sensor failed to show a richening response it added more. This cycle went on until the computer reached its max fuel enrichment strategy (there will be a max because too much fuel can wash down the cyl walls and allow the rings to scuff the cyl). Once that max point was reached and the sensors still didn't show a rich reaction, it threw the CEL code.
Recommendation: Replace both O2 sensors. Stick to the factory brand too (Denso I believe).