What causes a liquid sqeezing sensation that sounds like pop rocks and lasts 2-3 seconds at base of skull?

The sensation you're describing could be related to a phenomenon known as "cervical crepitus," which occurs when gas bubbles in the synovial fluid of the joints in the neck collapse or release during movement. This can produce a popping or crackling sound, similar to that of pop rocks. Additionally, muscle tension or spasms in the neck may contribute to this sensation. If it persists or is accompanied by pain, it’s advisable to consult a healthcare professional for further evaluation.