My money is on 3D Secure, a security protocol that requires card authentication before the card can be charged. 
https://stripe.com/docs/payments/3d-secure

So those "Redirect" orders are probably customers that were redirected to the authentication page but did not complete it.

Code seems to support this hypothesis:

https://github.com/craftcms/commerce-stripe/blob/b881fdde3a8d7253ae24c77ffd6feded0a91f4aa/src/gateways/Gateway.php#L535-L548

https://github.com/craftcms/commerce-stripe/blob/b881fdde3a8d7253ae24c77ffd6feded0a91f4aa/src/gateways/Gateway.php#L374-L382

https://github.com/craftcms/commerce-stripe/blob/b881fdde3a8d7253ae24c77ffd6feded0a91f4aa/src/responses/ChargeResponse.php#L89-L95

https://github.com/craftcms/commerce/blob/dd4822f2536aa6818acc0134a4a2e4b1b7d5dfef/src/services/Payments.php#L682-L683