Yeats' strange deviation from the subject in Easter, 1916

Hearts with one purpose alone
Through summer and winter seem
Enchanted to a stone
To trouble the living stream.

The horse that comes from the road.
The rider, the birds that range
From cloud to tumbling cloud,
Minute by minute they change;

A shadow of cloud on the stream
Changes minute by minute;
A horse-hoof slides on the brim,
And a horse plashes within it;

The long-legged moor-hens dive,
And hens to moor-cocks call;
Minute by minute they live:
The stone's in the midst of all.