Why is halving the best time for block size increase hard fork? Gavin Andresen said the best time to launch the block size increase update / hard fork would be the next reward halving (when mining discovery bounty would go from 25btc to 12.5btc).
Why has halving anything to do with increasing odds of success of a hard fork?

Source: Gaving Andersen said that at the Dev Core conference London on April 19th in his keynote "Why we need a bigger chain". There was a live broadcast of the event but I can't find a video link.
There's definitely no reason for that from a technical perspective. Success is entirely dependent on convincing all the wallets, nodes, and lastly miners in the network to make a breaking change to their software.