We can't know. you have not provided any proper technical details like system info, render settings, comp settings, footage and effects involved and whatnot.
I can answer your question if you can answer this one. "Why is watermelon?"
If you can't, then please tell us everything you did to create your composition, the settings, the footage used, the OS you are using and the version of AE down to the last decimal point, and your render settings. Without details we are missing the rest of the question. Without the rest of the question we can't help much.
Oh, My Fault, I Missed The Memo About How To Properly Ask A Question... I Hope You Guys Don't Work For Adobe Support... I'll Just Call And Ask Somebody. Thanks.
Technical support is going to want to know those questions too. It could be any one of 1 million things. No sense in guessing we don't know.
We seriously can't help without more information. What is in your comp? (Effects, footage codec/format, etc.) What were your render settings? Etc., etc., etc. We don't even know which version of AE you're using. On CC, you can use 11.x, 12.x, and 13.x (and the numbers in place of the x matter a lot for troubleshooting almost more than the major version number).
Oh, My Fault, I Missed The Memo About How To Properly Ask A Question
No problem! Here it is: FAQ: What information should I provide when asking a question on this forum?
It will help anyone trying to answer your question to have this sort of information (whether they're official Adobe support or not.)
I Hope You Guys Don't Work For Adobe Support... I'll Just Call And Ask Somebody. Thanks.
They don't; like me, they are just people volunteering their spare time in an attempt to help others.