That happens with mine before they fall off. Then there's a flush of new leaves/fronds. Although, I gather you'd be in the middle of your growing season so I wouldn't really expect that to be the cause . Is the plant in the ground or in a pot, and which species? Could be its soil is too wet, if that's the case it's a big problem. You need to improve the drainage.
All the Cycas I have/have seen go through the process of dropping all their leaves before the new flush, except my C. revoluta. But its behaviour is bizzare and unpredictable anyway. The only Zamias and Bowenia I have only lose fronds bit by bit and not 'en masse'. And the couple of Macrozamias I'm familiar with don't do it, to my knowledge.
If it's not one particular plant but is generally affecting a lot of them (and no apparent drainage problems) then perhaps it is part of their natural cycle.
I understand you have a lot of lime in the soil there. Maybe that's causing some sort of imbalance. Only guessing, of course, but the answer lies somewhere.