"A game is being started"
'Mchezo' is a MI- class noun (called this because the plurals are formed by adding mi-: 'michezo'). MI- class nouns take the u- subject prefix in the singular (and i- in the plural).
Thankfully, memorizing noun classes is less tedious than it sounds. With some practice, you'll start to pick up the patterns. :)
I am also confused about it. So, 1. as already asked, why unaanzawa and not anaanzawa if this is a (passive) verb? I can't find this in the tips 2. I did not get the explanation by kshuck2... So far from this course I have learned only about the adjective agreement in MI- class (vowel mw-, my-, and consonant m-, mi-). So, please, give further explanation ( :