For some reason, an RTO congestion signal would not set up CWR to be sent out, while reducing cwnd down to a single MSS, as per RFC. DCTCP had addressed this particular problem before, but this is generic to all ECN-enabled flows, thus changing the CWR flag in the generic cc cong signal wrapper.
This can help improve performance after an RTO, as some stacks will use the CWR flag as hint to perform an immediate ACK, rather than having a 50/50 chance of running into a delayed ACK timeout, before the receiver sends out an ACK to grow cwnd to 2 MSS again.