Why was the Oct 2015 Transaction Malleability event possible in spite of BIP62/66?
Canonical DER signature implemented in BIP 66 fixes issue #1 of BIP 62 ( Non-DER encoded ECDSA signatures ) 

Amacilin's code exploits issue #5 in BIP 62 ( Inherent ECSDA signature malleability ),  and is explained here : https://github.com/bitcoin/bitcoin/commit/a81cd96805ce6b65cca3a40ebbd3b2eb428abb7b 

This issue was fixed by requiring signatures to have low-S encoding in pull request 6769 : https://github.com/bitcoin/bitcoin/pull/6769 . Note that this fix only prevents mutated transactions from being relayed (transactions without low-S encoding can still be mined into blocks).