No real mysteries here. Could be computer glitch, could be things winked out briefly while new flights/fares are being loaded into the system, etc. As for flight options disappearing entirely and staying gone, it could be because their information is showing the flight has been cancelled for that day/time, or the flight is sold out. If a flight has disappeared on more than one booking engine, then changes or updates from the source (airline provided info) is probably the cause. April 3-5 is a 3-day holiday in China, and flights into KWL are not that plentiful, so it wouldn't surprise me if flights on the 3rd fill up. I wouldn't trust or use Expedia.com for domestic China bookings, and real-time updates to their system may be lagging behind.
I have had success with both ctrip and elong, you can also see if travelzen.com has the flight you are looking for online. Call ctrip's English line for US customers and do a phone booking.