Sforno on Exodus 14:25/ספורנו על שמות י״ד:כ״ה

1ויסר את אופן, by means of the column of fire.

2בכבדות, due to the heavy mud in which they were sunk.

3כי ה' נלחם להם, they were hoping that once G’d saw them fleeing He would desist from fighting them.