The old woman across the way is whipping the boy again (1-2)
That woman is whipping the boy again. This is our first indication that this is happened before and the poem's first mention of the idea that the past repeats itself.
His tears are rainy weather to woundlike memories: (11-12)
The past really can hurt sometimes. Here, somebody's memories (probably the woman's, but they could also be the speaker's) are like "wounds." Apparently, they haven't healed yet either.
My head gripped in bony vise of knees, the writhing struggle to wrench free, the blows, the fear worse than blows that hateful (13-16)
These lines more than anything else show how painful memories come roaring out of the darkness of the unconsciousness. The suddenness with which this little stream of consciousness interlude shows up imitates the suddenness with which painful memories pop into our heads out of nowhere, sometimes.
And the woman leans muttering against a tree, exhausted, purged— avenged in part for lifelong hidings she has had to bear. (23-24)
The woman has had to hide her traumatic past, and it has made her life absolutely miserable. That's why she's so violent—she's angry about the past, tired of carrying that burden around, and thinks she can "purge" herself of that pain by whipping some poor kid.