No I guess you're right, there's no possible way that they wouldn't have assumed the guy who never opened fire on them while they were killing billions would ever decide to attack. /sarcasm.
Seriously though the gk have to be fairly stupid to not expect an attack.
There was little meat to the reports, and nothing in the way of actual conflict. The Wolves never waited for long, nor did they return fire when fired upon. Their desire was to evacuate every soul they could, and scatter them from our paths.
Doesn't sound much like a group who are prepared to attack you, does it?
The Grey Knights realised the Wolves' game earlier than the inquisitors holding our leashes. On more than one occasion in hololithic transmission meetings, I'd been forced to watch Lord Inquisitor Kysnaros shouting down all counter-arguments offered by our order. He was still certain containment could be reached.
The Inquisition didn't think the Wolves would attack, and therefore ordered the Grey Knights to continue regardless of the increasing likelihood of a Space Wolf attack. Granted, the Grey Knight leader didn't really help matters (it's noted in the book that he 'lacks a great many commendations for command'), but he was also bound by the command of Kysnaros.
'Kysnaros suspected the Wolves of treachery. We opened fire before they had a chance.'
'And you believe that, my lord?'
He had the aduacity to laugh, despite the gravity of the situation. 'Not for a second. But this is our chance, brother. We take Grimnar captive, and his Chapter will kneel in submission'.
'This is perfidy, Joros. It leeches any honour our order ever laid claim to.'
Castor shook his head. 'Signal's lost, sir.'
After the Inquisition issues the message to the Wolves about coming aboard to discuss terms, Hyperion says "I honestly expected them to die in proud silence, aboard their wounded flagship."
They did not expect an attack from the seemingly crippled and until-now-peaceful Space Wolves.
The Space Wolves agreed to come aboard to discuss terms, and Brand Rawthroat even told Hyperion to 'watch how a Wolf surrenders'.
While he lacked a great many commendations for command, a a duellist and a front-line fighter, it was acknowledged across the order that few could match his reputation and skills with two falchion blades. A vital aspect in any blademaster's repertoire is the ability to read an opponent's movements, and react with greater speed than they can act in the first place. Joros was a master, and his reflexes were renowned.
And yet, his blades had scarcely cleared his scabbards when Logan Grimnar's axe of blackened steel and burnished gold cleaved into our Grand Master's breastplate and throat, ending a worthy, respectable life of service with a single crunching chop.
Joros went down, felled by the axe blow and dead before he hit the ground... In the time it had taken me to look back from Rawthroat to his liege lord, my own Grand Master was slain. That should explain, at least partially, how quickly the High King of Fenris moved.
Now that's what I call a 'surprise attack'.