I would consider this a bug but if it was designed this way then it's a flaw.

As all guilds were attacking castle when it opened, it was changing hands over and over.
Some of the attacks landed but the teams got sent back to fortress and we figured out what was happening.

During a long exchange, if you launch at castle and you own it, the game considers that as camping / reinforcing.
Before it lands, if another guild takes castle, that guild owns it.
When your troops land at castle, instead of attacking the castle, it still believes you own the castle and tries to camp there and fails and you're sent back to fortress.

This is a serious bug (or design flaw) since there's no shield on castle. if everyone is attacking it at the same time, it should be treated as an attack even if you had launched as reinforcement since another guild had taken possession.