In hardwoods, burls are knobby warty overgrowths caused by response to some sort of damage - often fungal infection. The woody growth walls off the pathogen and protects the rest of the tree.
Burls are a normal thing for redwoods. Most big ones will have some burls. They are also knobby growths but serve a different purpose for these trees.
Burl is the knobby growth most commonly seen at base of some coast
redwoods, though it can also be found high in the canopy as well. Burl
is a woody material full of unsprouted bud tissue. It serves as a
storage compartment for the genetic code of the parent tree. If the
redwood falls or is damaged, the burl may sprout another redwood tree
known as a clone.
The burl is a fallback defense in case the tree falls down. It can sprout back from one or more of its burls.
So too your trees. Once they get to a certain height and are at risk of being toppled, each develops a big burl. If the tree falls the burl kicks in growing. A burl is bigger than any seed around and can rapidly reclaim the hard-earned space in the forest before that space is filled in by upstart seedlings.
Galls are similar to burls but smaller, and caused by insect pathogens. The insects hijack the tree growth mechanism to make themselves an armored chamber.
Growth deformities known as galls commonly occur on oak trees in
Texas. Most oak trees are attacked by a group of small insects called
gall makers. These insects can cause deformities, called galls, in the
leaves, twigs, bark, buds, flowers, nuts, or roots of the tree.
Because of the unusual size and shape of the horned oak gall and the
gouty oak gall, they attract the attention of landowners and
homeowners who are concerned about what causes the galls and what harm
or damage they will do to oak trees.
Your trees have giant galls. Once they get to a certain size, something moves in and hijacks the growth of the tree, forcing it to create a gall-like chamber. The growing thing feeds off of the tree sap. Maybe these things stay in their tree, releasing spores or seeds or motile forms which go off to find new trees. Maybe these things break out once they reach maturity, leaving a gall with a big hole and a space inside. Some real galls have holes where wasps break in to lay an egg on the caterpillar inside. Your giant galls might have holes for the same reason - something wants to get at the creature living inside.