She's caught him completely off guard. Their game is usually chasing (or stalking) so her leaping in attack - a playful one regardless - isn't something Hati is expecting.
And she's so small, knocking him over is all but impossible, so he'll catch her instead, while Sköll walks off in mild disgust. Hati looks at her face for a moment before carrying her over to the nearest pile of snow and gently enough dropping her into it.