Leakage through ceiling in basement unit, is this due to weather?

[Kevin Huang]

Tenants living in the bottom unit (below ground) mentioned that there appears to be some leakage going on.  I went to take a look and it looks like there's water coming through the ceiling.  I'm not totally sure where this water would be coming from as the area showing the water damage is directly beneath the floor of the upper unit, but I have a hunch that whatever it is, it has something to do with all the snow/ice/etc.

Asking folks with more experience than myself and have lived through Chicago winters if they have any guesses as to what the issue might be?

Some facts that might be helpful:

-I have a flat roof

-The damaged area located right against the back wall of the building, very close to the vertical gutter

-The unit directly above has experienced water leaking through the top of their door and window frames as well