Im only moving my tank to the grass for the 59% part. I'm only doing it with 6 toons, most all non donor so I doubt my dps is very high. I don't use any pets so the 1st person he wants to summon is the tank, but just like you are seeing with the pets he is summoning before the DT check goes off.
He does stop summoning when the 59% part begins, but resumes summoning too quickly.
Ive seen it happen to lots of other people too but most tanks use the DI aries shield and I guess dont event notice it. I know I have lived through it just from the raid aura DI several times, but its only like 40% chance so I still get killed by it a lot. I use Mary to tank him until I get Takin resed. This usually results in the rest of my dps/debuffers getting killed as well from the change in agro.