Why was 95% adoption rate selected for activation as oppossed to 80% or 50% or even 5%?
So that the soft fork deploys with supermajority.
It will essentially have consensus when it deploys. This ensures that the new rules will be deployed and enforced by the miners. The 95% rule has been used for all previous soft forks, no reason to change that now.
The consensus would be among the miners, and not necessarily among the entire Bitcoin economy. Just because 95% of the blocks that the miners produce doesn't mean that non-mining Bitcoin related companies and other Bitcoin users support SegWit (Note: ready to implement != support).
Technically speaking, the threshold is 50.694% (or potentially less depending on luck) for activation as the miners who support SegWit could decide to ignore blocks that lack a signal of support for SegWit.