Does the sentence
"Experts are unsure about what caused the increase in suicides to be so disproportionately steep among Native Americans and white people, during a period which spanned the recession"
"Experts are unsure about what caused the increase in suicides which is so disproportionately steep among Native Americans and white people, during a period which spanned the recession."?
What are the different usage of this type of "to be"?
It's a common usage.
This book used to be blue. Now, it is red.
What caused it to be red?
I covered it in red paint.
Look at my hand. It's black and blue!
What caused it to be black and blue?
I dropped a 30-pound weight on it.
What caused birds of paradise to be so many beautiful colours?
Scientists aren't sure what caused them to be so many beautiful colours. It might be to attract a mate or to make sure they can see each other in the dense foliage.
In your original example, your interpretation is pretty much right, although the original sentence is more natural. In your version, you just need to change "which is" to "which was" because the piece makes it clear it is talking about a period in the past (the period which spanned the recession).
Remember - if you don't use correct capitalisation, punctuation and spacing, anything you write will be incorrect.