What can cause a root canal crack?

Excess Force: Excess forces on a tooth can cause an already brittle tooth to fracture. That's why crowns are indicated for teeth that have had root canal therapy.

Root canal crack: Tooth become brittle after root canal treatment. If the tooth is not adequately restored with a filing, cap or crown the remaining tooth structure will be very prone to crack with the result that the entire tooth may have to be extracted.

A Post: A post placed into the canal after a root canal can cause the root to crack. The post is placed to help anchor the crown if there is not enough tooth structure remaining. The post does not strengthen the tooth; it actually weakens the tooth. Undue forces applied when inserting a post aggressively can cause a crack.