I have had similar problems. A partially blocked inspection pit outflow - caused the water level to rise in the pit. A sudden inflow from an upstairs bath increases the air pressure in the pit and this forces water back into the house - the pit slowly drains through the partial blockage.
Find the inspection pit cover. Old clothes - rubber gloves - hosepipe and a poking stick. Ten minutes poking around and another ten clearing up. Sort of job best done as soon as you notice it - less foul water in the pit.
Peccavi
December 2008