RMT Basics, page 8

You've Got Leverage

Most material things break first in tension because leverage is involved.

Under circumstances where the potential for leverage is insignificant, most material things break in compression, unless stretched to the breaking point. The reason is simple:

Most material things are stronger in compression.

Consequently, stretching aside, whether a certain material thing breaks first in tension or breaks first in compression is most often determined by the degree of leverage involved in breaking it.