I was recently diagnosed with trichomoniasis. I have been in a faithful and monogamous relationship for over a year now. How else can this hhappen?

Trich. There are a couple of scenarios. First, you may have brought it in from a previous relationship if you had one. Or your partner may have given it to you.
It can be a "Silent" Std. Trichomoniasis is an std caused by a parasite called trichomonas vaginalis. Approximately 70% of people w trichomoniaisis infection are asymptomatic. Thus, you or your significant other may have had the infection beforehand ; have had no idea.