In Act 3, Scene 4, Why does Iago discuss the nature of Othello’s anger in Lines 126-134?

Line numbers don't always match up from book to book. The following seems to be what you are referring to,

Iago:

Can he be angry? I have seen the cannon,

When it hath blown his ranks into the air,

And, like the devil, from his very arm

Puff'd his own brother:—and is he angry?

Something of moment then: I will go meet him:

There's matter in't indeed, if he be angry.

Iago says that he has seen Othello handle himself in the heat of battle get if he is truly angry, it is over something very serious.

Here is this last part again without the auto-spell correct!,

Iago says that he has seen Othello handle himself in the heat of battle. He says he has seen men blown away with cannons and his own brother die in front of him, yet he has kept his cool. If Othello is truly angry, it is over something very serious.