They are besieging Nuln, endgame and close to their prize.
The siege has gone on long enough for the besieged force to have lost 45% of their number.
This is their endgame, if they lose here it's a mop-up operation for us.
This isn't a campaign we started. This is a campaign they started, we just arrived for the final battle(Objective in sight: Kill Magnus).
Enough blood has been shed for them to even summon an extra ???, something that took them an unknown number of years grinding in warped time and a worthy sacrifice the last time.
Also, there's a near point parity in troops between individual sides while we have the fortifications in a free-for-all.
Of course there's going to be a second/third wave if we choose defence, it's why we should aim our cannons mostly at the Chaos forces and fort up.
We killed a third of the Ork forces, of the two, they are less liable to be reinforced with anything huge and unlike Thorgar, they just want to fight, not specifically kill Magnus.
The disappearance of the Juggernauts alongside our failed cavalry charge also implies the Orks have good anti-cav preparations so I'm not horribly worried about chariots, especially considering us having fortifications.