Pain and tightness in Achilles after wearing birkenstocks. What is cause?

Low heel shoe. Birkenstock sandals usually have low heel cushion meaning your heel is closer to the ground. As a result, there is tension on your achilles tendon. If you normally wear dress shoes or heels then it will really be tight due to the achilles tendon being shortened and contracted. Important to remember to stretch your calf muscles.
Over stretching. If you are used to wearing an elevated heal the birkenstock may have a neutral or negative heel over stretching the achilles tendon and creating pain.