DLM doesn't change the interleaving depth directly. It changes the INP and delay values ... and the modem chooses (during sync) an appropriate interleaving level to suit those requirements. It can alter independently (at the next sync) if there are other environmental changes, such as a slight change in sync speed.
In the absence of any change to INP and delay, the interleaving level probably changes at the same time as the interleaving block size, the RS block size, and the number of parity bytes carried in the RS block.