Okay well it is possible if any of these were the circumstances:
Your Hydreigon was Paralysed. Being Paralysed reduces your Speed Stat by 50%.
You were against a Ursaring with Quick Feet. If the Ursaring was Burned, Paralysed or Poisoned Quick Feet will Boost it's Speed Stat by 50%.
Ursaring could have used Scary Face on your Hydreigon. Ursaring can learn Scary Face at Level 38 in Pokémon X and Y. Scary Face reduces your Speed Stat by two stages. Since Ursaring can only be caught at either Level 50 or Level 52 in Pokémon X and Y at Route 21. It only learns two other Moves between Level 38 and Level 50/ 52 so if you catch a Ursaring in Pokémon X and Y they will always have Scary Face.
Hope I helped :)