At least they put them on a little raft...On more than one occasion,i have seen where the floor boards,or at least the short section that invariably bridges these JB nests,is resting on the JB cover tops.
If they are packed with conductors,the covers are sometimes tight against these,and just the constant tiny deflection of foot traffic,above,usually near a doorway,can turn them in to a fault-pressure pad
In the past,i had one in a large Victorian semi,where everybody had tried to find this random tripping issue,and given up.
After standard tests had not revealed much,apart from confirming the mess of it,i had a lad marching round the place,with an IR test locked on a group of circuits, half an hour later,a bedroom doorway,was discovered to be the display changer
Carpets and boards up,and it looked just like your picture
