Discusion question: Whats with this meter break in" The Rabbit- Hunter" Robert Frost

[kolemath]

A few other lines aren't iamb, in bold

The Rabbit- Hunter by Robert Frost

Careless and still
The hunter lurks
With gun depressed,
Facing alone
The alder swamps
Ghastly snow- white.
And his hound works
In the offing there
Like one possessed,
And yelps delight
And sings and romps,
Bringing him on
The shadowy hare
For him to rend
And deal a death
That he nor it
(Nor I) have wit
To comprehend.

---

I have read this over and over today. I was using it to try to learn meter. I think this is mostly in Iambic Di-meter but two lines in bold are 2.5 feet. I can not for the life of me figure out why he would have done that and I was hoping maybe someone else would have a clue. Any guesses would be greatly appreciated; I am sure he didn't just do it for the hell of it- maybe... Thank you!

Frost said to pick a meter for your theme then write words to fit the meter so there's prob a reason

1, 4, 6, q12, 13    
dactylic for doubting the kill?

7,8 anapestic
Alter the rhythm for the dumb dog?

 Iamb for killing?