Hi Ben,
1. The output data of ADXL375 is formatted as twos complement data. When high 8-bits are all one, that may also mean a quite small negative acceleration.
2. When you use multi-byte read, please make sure your sequency is:
CS is asserted->send read bit -> send MB bit -> send 6 bits address -> send 8 clock for reading DATAX0 -> send 8 clock for reading DATAX1 -> send 8 clock for reading DATAY0 -> ... -> CS is deasserted.
Hope these can help.
BR
Neil