I’ve noticed recently in Spain that more and more trucks seem to have there hazard flashers on when going up hills. Is this some sort of new legal thing or just a bit of a fad among drivers ?.
Its more of a personal safety thing, the average spanish car driver has no idea what is past the end of the bonnet of their car. they drive flat out everywhere (since monobrow alonso was world champion its worse 
) and when its wet seem to go even faster 
they go uphill at 110/120 kph and the trucks are doing 40 and at the very last minute they suddenly see the truck and either swerve or slam on the brakes, even on a 3 lane motorway 
some might be reading this and not understand why we go so slow uop the “hills” but the OP has enough experience of spain to know why 
the climb out of malaga is over 800m which translates to something like 2500ft (ish) thats a big climb but not when you get to granada 1380m or somosierra 1444m now even big motors like scania V8’s and volvo FH16’s need to use the gearbox on some of those hills.
I’m not comfortable using haz flashers whilst moving but it beats alonso junior in his seat leon fr with his girly sunglasses running up your arse
WBIS,i am betting that i am not the only one to pretend to be an airline pilot,while descending those big hills in Spain,in a moment of madness i would talk to myself,adjusting the controls in the cab,like adjusting the flaps to prepare to land.
Its the law in Spain (I beleive) that HGVs travelling up or downhill at speeds below 40kph have their hazards on.
As WBIS says some of the up/down hills here are not only steep, but very, very long - 10kms is not unusual.
I first saw that in the US - a sign on a reasonably steep climb on I-80 in Pennsylvania made me laugh: TRUCKS USE 4-WAYS UNDER 50 MPH