Don’t know exact cause, experience same thing from time to time, usually it all automatically corrects itself within few hours or a day after publishing a new product or updated an existing one. It probably has something to do with how the database progressively updates and catalog the new content.
If images or a product page keep disappearing after a day, then its provably best to inform CGtrader staff about it.
PS: Avoid keep trying to update or re-upload content, it will prolong the problem.