RAW: time stop only stops time for creatures, not physics/the environment
The time stop spell description states:
You briefly stop the flow of time for everyone but yourself. No time passes for other creatures, while you take 1d4 + 1 turns in a row, during which you can use actions and move as normal.
This spell ends if one of the actions you use during this period, or any effects that you create during this period, affects a creature other than you or an object being worn or carried by someone other than you. In addition, the spell ends if you move to a place more than 1,000 feet from the location where you cast it.
Jeremy Crawford addresses a related question on Twitter here:
If you cast Time Stop and cut a rope bridge does the bridge fall while the creatures on it stay?
Time stop ends, as stated in its description, if you do anything that affects other creatures. Causing them to fall counts. Bye, spell.
But nothing happens to them yet? The bridge isn't going to fall, because time is stopped.
Time resumes because you did something that affects other creatures.
This indicates that environmental effects such as gravity (and the severing of a rope that holds a bridge up) continue to take effect as normal, and do not wait for you to finish the extra turns granted to you by the time stop spell.
Thus, any action you take during that time will cause the spell to end early if the action or its effects would affect a creature besides you (the caster). Crawford's ruling also suggests that this is true even if you don't directly affect them; affecting the environment in a way that indirectly affects them still causes the spell to end.
...but as a DM you (and Jeremy Crawford) might rule otherwise
Crawford followed up that last tweet with one that suggests the exact opposite:
Alternative take: Cut a rope bridge, and nothing happens until time stop ends. This is most likely how I would run it, Mr. Crawford.
This seems to be a more straightforward interpretation of the spell. Crawford says he'd probably rule as DM that time stops for everyone and everything, including the environment. Given this interpretation, the caster of time stop is much more free to set things up such that the target might be hurt or killed by environmental effects as soon as time resumes; according to this, the spell wouldn't end early unless you directly affected another creature.
Conflicting rulings suggest that either interpretation is reasonable
The fact that Crawford presents two differing interpretations (one RAW, one based on how he'd personally rule as DM) suggests that either interpretation is fairly reasonable. The important thing is to be consistent as a DM in how you rule it, and to make the mechanics of the spell clear to anyone who's considering learning the spell.
It's also important to be aware of the implications of whichever ruling you follow. If you allow the caster (per the latter ruling) to do things that can indirectly harm the enemy without causing the spell to end early, it will potentially make the spell much more powerful.