Leaf drop is one of the most common plant complaints there is, and unlike a lot of plant problems, it affects indoor and outdoor plants in largely the same handful of ways. This is a general guide to leaf drop across any plant; if you're specifically dealing with tomatoes turning yellow before dropping, this site's dedicated tomato yellow-leaf guide works through that crop's particular causes, from natural ageing to blight, in more depth than a general article like this one can.
Environmental shock is one of the most common causes, and it catches people out because it's not obviously connected to anything they've done wrong. A recent move, a shift from a garden centre's controlled conditions to a home windowsill, a sudden draught from a door or window, or a sharp temperature swing can all cause a plant to drop leaves within days, even when watering and light are otherwise fine. The plant isn't reacting to a long-term problem, it's reacting to a sudden change in its conditions, and it often settles down and stops dropping leaves once it's had a few weeks to adjust to its new spot.
Watering is the other major cause, and the genuinely useful, often-surprising point here is that both overwatering and underwatering can cause leaf drop, not just one. It's tempting to assume dropped leaves always mean a thirsty, underwatered plant, but waterlogged roots that can't take up oxygen properly cause just as much leaf drop as roots that have dried out completely, sometimes more. Checking the soil itself, rather than assuming which extreme you're dealing with from the leaf drop alone, is the only reliable way to tell the two apart, since both can produce a similar-looking result.
the genuinely useful, often-surprising point here is that both overwatering and underwatering can cause leaf drop, not just one.
Finally, it's worth telling normal seasonal leaf drop apart from a real problem. A deciduous plant dropping its leaves in autumn and winter is doing exactly what it's supposed to do, and no amount of watering or feeding will stop it, because it's a seasonal cycle rather than a sign of stress. An evergreen plant is a different story: it should hold most of its leaves year-round, so any evergreen dropping leaves heavily, outside of the odd old leaf here and there, is genuinely worth investigating, rather than assumed to be normal seasonal behaviour the way it would be on a deciduous plant.
If a plant is dropping leaves, work through these in order: rule out a recent change in its environment first, check the soil to see which watering extreme you might be dealing with, and consider whether the plant is a deciduous type doing something completely normal for the season, before assuming something's seriously wrong.