Tree burls can be caused by a number of things.
Here is the ones I know, feel free to add others I have missed.
wound (including those from animal claws)
insects
virus
bacteria
fungi
Some of the more distinctive growths, can be identified to a particular thing that caused the growth:
Bacterial Crown Gall (Agrobacterium radiobacter)
Poplar Budgall Mite (Canadian Forestry Service) (Aceria parapopuli)
Corky Bark Disease (Diplodia tumefaciens)
How should I identify burls that the cause can’t be determined?
The tree species?
I can’t seem to find any other identification in iNat.