Liquidation can happen anytime (before or after the maturity date) if the loan to collateral ratio below the liquidation threshold.
There are two type of triggers of liquidation: 1) the loan to collateral factor is larger than the liquidation threshold. 2) the loan position is overdue.
Once the loan become liquidable, any users can call the liquidation function to liquidate the loan position. Learn more details regarding the liquidation here (https://docs.definer.org/liquidate).