Levain did't double in size when kept at 85º??

I make the Tartine Country Bread.

My levain is 150 grams water and 150 grams flour (flour is half whole wheat and half all purpose) and 20 to 30 grams of starter.

 I usually keep it on my counter at 70º and it will double in about 11 hours and this gives me a good indication that it is ready to use.

This time I kept the levain in my proofing box at 85º (to speed up fermentation) for about 7.5 hours and it didn't double.  It was very loose and liquidish in when I used it.

I don't understand why the levain didn't not work right when I kept it at a higher temperature?

Thank you.