What is 'natural aggregation' and 'workers' comp'?
Insurance companies have long tried to avoid natural aggregation, such as limiting homes insured on a shoreline.
Now, Buffett asserted, companies need to think about manmade aggregation risks.
As an example, while most folks think of the World Trade Center disaster as primarily a property/casualty loss, Buffett noted it will easily go down as the largest workers’ comp loss in history.

Question

What is 'natural aggregation' and 'workers' comp'?
Insurance companies have long tried to avoid natural aggregation, such as limiting homes insured on a shoreline. 
Now, Buffett asserted, companies need to think about manmade aggregation risks.
As an example, while most folks think of the World Trade Center disaster as primarily a property/casualty loss, Buffett noted it will easily go down as the largest workers’ comp loss in history.

Answer

I can't help you with natural aggregation.  I did a search online and didn't find anything useful.  It sounds like a technical term in the insurance industry.

Workman's compensation is a U.S. government program that provides financial assistance to workers who are injured on the job (injured while working).  Here's some information from the U.S. Department of Labor, which administers (oversees) the program:  https://www.dol.gov/general/topic/workcomp

Most Americans, I think, would call it workman's comp., but it looks like the Department of Labor has changed the name of the program to make it gender neutral.

Answer

Yeah, this one is tricky. The verb aggregate most commonly means to group, or to filter. A large data sheet sorted by a given variable is described as aggregated, or filtered. Insurance companies spend a lot of time running statistics intended to predict things, namely how likely something is to be destroyed. If a house is in a flood zone it is naturally aggregated because there is always a risk of a flood.

Seems like the next point in the article is that insurance companies will have to recalculate how they predict the likelihood of commodities being destroyed based on man-made phenomena (like acts of terror), instead of just natural ones (like floods).