What should I do when the imam of the masjid starts doing innovations? I cannot leave because there are people in the back rows, and it is troublesome to go past them.

All perfect praise be to Allah, the Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad, sallallaahu ʻalayhi wa sallam, is His slave and Messenger.

It is possible that what you think is an innovation is not actually an innovation, and it might be an innovation according to some scholars but not to others. In any case, do not judge that a certain action is an innovation except with firm certainty by asking reliable scholars. If you become certain that it is an innovation, you should give advice to this imaam and forbid him from committing it to the best of your ability. If he refused to accept your advice and you could not leave, then you bear no sin, because you did what is within your ability to do, and Allah does not burden a soul with what is beyond its ability.

Allah knows best.