What caused the start of the ice battle?

The knights of the Teutonic Order were going to seize the Novgorod lands (ideally, the plan provided for the capture of Novgorod itself, but initially it was supposed to occupy Pskov and the surrounding area). Moreover, at that moment Russia was attacked by the Mongol-Tatar army. And she was almost drained of blood.
However, the Novgorod squad, which did not participate in repelling the attacks of the Mongols, together with the assembled militia, opposed the knights. In order to prevent the devastation of the land on the eve of spring and summer, when it was required to begin the beginning of earthworks.