Tail Chasin' Horse

This evening, while feeding the herd, my 9 yr Morgan/Haflinger gelding (Tango) was in his paddock chasing his tail! He looked just like a dog! He did this for the first time last night...and then again tonight. It was so strange to see, I couldn't let it go without making a remark about it! He'd chase his tail, then stop to give a small buck or two, then continued on chasing his tail!

I thought maybe he was pissed because he's always the last to be fed and his sick and tired of it! Once I feed him he's fine. He's never displayed this behavior before last night.

Anyone have a clue to what's up with this?