latency at scale

I believe I am lacking some knowlege regarding this. There are 10 pods of my service running in production. We saw a huge scale today and everything was mostly fine. But as soon as we started reaching 200k / min cpu increased normally ( I think) but suddenly memory started fluctuating a lot but still remained within 300mb (4gb available) and p99 started rising to above 1000ms from normal of 100ms. Given cpu and memory were mostly fine how can I explain this ? Service is simple pass through takes a request and calls downstream service and returns response.