Is it because double-spends are now easier?
Yes.
Why are they easier?
Because BIP91 requires miners to orphan blocks that do not signal BIP141 (SegWit), with the result that any transactions confirmed in such a block will become unconfirmed again, potentially allowing double-spending. The actual risk depends on how many miners are actually enforcing BIP91 and how many are signalling BIP141, which is difficult to determine with any certainty. As long as a significant number of miners are enforcing BIP91, non-SegWit blocks will probably be orphaned before getting a large number of confirmations, but transactions with a small number of confirmations are at risk of becoming unconfirmed at any time, and should be treated as unconfirmed transactions until they have many more confirmations.