Why does my minecraft server keep crashing?

Too many mods, players? Check your router… Hard to guess with little info

Your computer is not good enough

If you have too many players on, for instance people venturing away from spawn, and making new world, well that has to load brand new chunks in, which will in turn lag the server, and if enough people do it, it can crash the server. Also, items. If you have a mob grinder, and have all these dropped items, that can crash your server. Animals. Spawning a ton of animals? I mean 500+++ animals? Your server will crash.

Anything that gets to be too much, normally will lag the server, or it will crash the server.