Unique champion takedowns grant permanent out of combat MS.
Fleet Footwork is a strange choice for jungle Sejuani, but the extra bit of healing keeps you extra healthy for ganks and the burst of Movement Speed in duels allows you to stick to your target to build [E] stacks.
Triumph on front-line tanks allows you to absorb an incredible amount of damage for your team and continuing fighting even longer after every assists thanks to the massive Triumph heal.
The extra bit of Attack Speed from Alacrity is help in a faster jungle clear and more importantly, in building [E] stacks extremely quick.
WhyCoup De Grace
Coup De Grace is the go-to rune for reliable damage amplification, especially considering you're a tank and will have a hard time getting any benefit from the other 2 options.
Zombie Ward is an all-around great rune choice especially when paired with the warding smite. The extra amount of vision it grants massively helps to keep your laners safe.
The bonus Movement Speed granted by Relentless Hunter allows you to get around the map for a faster clear and to respond to fights quickly.