A target mix is a written proportion: so much in stocks, so much in bonds, so much in cash, if those are the buckets you chose. Markets move the proportions without asking. Rebalancing is the act of trading back toward the page you wrote.
It is not a forecast. You are not claiming the thing you sold will fall. You are claiming that the mix you wanted last year is still the mix you want. If it is not, the task is to rewrite the page, not to disguise a new bet as maintenance.