Hi
If for each server you have 2 links towards the switch and one is up and another is down then i would suggest to check the following:
1. Compare the cable/optics of the working link with the non working link (for same server).
2. Check the auto neg settings on the qfx5100 and the state on the server.
3. Also check if there is any static bundling of the 2 links on the server side etc
Hope this helps