Maintenance for the week of April 15:
• [COMPLETE] ESO Store and Account System for maintenance – April 16, 8:00AM EDT (12:00 UTC) - 6:00PM EDT (22:00 UTC)
All siege engines constantly decay while they're out and it speeds up if they're not actively being used.
Even while continually using it to launch attacks, siege engines will still fall apart eventually if either you or someone else isn't periodically using siege repair kits on it.