The phrase "similar trajectory" means that two or more things are following a similar path or direction, or are likely to end up in the same outcome or result. Full definition
In that situation, ETH and ETC basically followed similar trajectories in terms of growth and losses, which is where I ultimately expect Bitcoin Cash to end up. (profitconfidential.com)
If Gaidos and his team are right, further collisions could have knocked free additional objects on similar trajectories, which means we may see more of this asteroid's cousins. (newscientist.com)
Bitcoin Cash, Litecoin and Ethereum have all seen similar trajectories in 2017, and indications point to continued growth. (links2exchange.net)