I think I understand Franky1's sense of security here, let me make a simpler example:

As soon as segwit code is out, some malicious miner create an illegal segwit transaction and mined it into a old style block, for example block 420001. All the old nodes will accept this block since they can not verify if the transaction is correct. So this block containing illegal segwit transaction will be berried deep down until the day segwit activates, lets say 425000

After segwit activation, all segwit nodes detected this illegal transaction (because they could not find the witness part at all) and illegal blocks,  they have to abandon all the blocks after this block and build from block 420001, since that is the latest block they consider to be valid, so network split into two chains from 420000 and all the transactions after 420001 disappear in segwit nodes after the activation