Remember Ichigo's fight against Kenpachi, when Ken stabbed his sword through Ichigo's into his chest, and said that it's because he relaxed.
When Grimmjaw was speaking with Aizen, of course he's not in a state of battle readiness. He was calm that time, and his defense was gone. Let's also not forget that Tousen is a captain-level shinigami, and he in Grimmjaw's unguarded state it's possible to cut him. I also believe Tousen had gotten stronger thanks to the HonGyokou