Hello Richard,
if the local node misses GLBP hellos of one of other node(s) it can change state as reported in the log messages:
000661: Dec 9 22:04:54.662 EST: %GLBP-6-FWDSTATECHANGE: Vlan138 Grp 1 Fwd 1 state Listen -> Active
000662: Dec 9 22:04:56.666 EST: %GLBP-6-STATECHANGE: Vlan138 Grp 1 state Active -> Speak
000663: Dec 9 22:04:56.666 EST: %GLBP-6-FWDSTATECHANGE: Vlan138 Grp 1 Fwd 1 state Active -> Listen
this can be caused by a saturated link or by high cpu on neighbor device.
You should investigate also on the companion devices that take part at the same GLBP group.
Hope to help
Giuseppe
