Are you diving dry? Put the spool(s) in your pocket. I try not to carry spools in the open unless it's for a very short trip. I've never had one come apart in the water on me though. I prefer delrin spools with a bunch of holes so I can add the right amount of tension. Halcyon makes them, as do Salvo and a couple of others. Dive Rite's are plastic and I don't use those, and DSS has metal ones which I don't care for, and their Delrin ones only have a couple of holes, so I don't use those either.
If the line is properly tensioned it really shouldn't matter too much if the latch opens. I have to work a bit to get the double ender out of the hole when I am ready to use the spool. Was this a brand new spool or one that you had used underwater before?