What authority has deemed this a phishing site?
CloudFlare does. Is says so right at the bottom of the warning message. CloudFlare is a CDN that TinyURL apparently uses.
It is a bit strange that CloudFlare would brand TinyURL as phishers, since obviously it is not TinyURL who is doing the phishing, but people using their service to generate obfuscated URL:s to dodgy sites. One could easily imagine a lot of their URL:s being reported as phishing, and that resulting in a block, though.
Has the URL name been hijacked?
I see no reason to believe that. CloudFlare can change the content of tinyurl.com because TinyURL has decided to pass all their traffic through CloudFlare.
It is possible that someone else has hijacked the site and that is the reason CloudFlare is blocking it, but I would not bet on that.
Maybe it is something that I've done or need to correct on my end?
No. This has nothing to do with you.
Is there cause not to go to tinyurl.com and create redirect links for my lengthy URLs?
I would not worry about my own security (anymore than I already do when clicking an obfuscated URL). I would, however, worry about the people I distribute the link to seeing that message and believing I am the phisher. Thats not good for PR...
But my guess is that this is some sort of mistake and the warning will dissapear shortly.
Edit: Now, an hour later, the warning page is not displayed anymore. Looks like my guess was correct - this was probably a mistake on CloudFlares part.