Aids to Writing Latin Prose: With Exercises. E-book. Formato PDF George Granville Bradley   -  Forgotten Books, 2017  - 

It is sufficient to call attention here to the tendency (not an invariable usage) of Latin writers to discard the use of the indicative in subordinate clauses where the verb on which they depend is on any ground in any mood other than the indicative; i.e. To introduce the subjunctive mood in relatival and other clauses on no other ground than that they depend on a verb in the subjunctive or infinitive mood. This tendency does not amount to a rule, but it will often account for the use of a subjunctive which would otherwise be perplexing.