【Explain】 ,  【account】 ,  【justify】 ,  【rationalize】  are comparable when they mean to give or tell the cause, reason, nature, or significance of something obscure or questionable.

One explains what is hard to understand because it is mysterious in its origin or nature or lacks an apparent or sufficient cause or is full of inconsistencies.

【Explain】 often implies an attempt to excuse or to set oneself right with others.

One accounts for something, rather than explains it, when one shows how it fits into a natural order or a logically consistent pattern.

One justifies himself or another when he explains certain acts or behavior in an attempt to free himself or another from blame. It may or may not imply consciousness of guilt or a definite accusation.

One rationalizes something that is or seems to be contrary to reason when he attempts an explanation that is in accord with scientific principles or with reality as known to the senses.

【Rationalize】  may come close to  【justify】 without, however, so strong an implication of blame and with the added implication of self-deception and, at times, of hypocrisy.