For that to be true then these guys would be hitching a ride and there should be more guys inside.For me the explanation is much more simple. They did it becouse they are quite far away from combat zone and it's much more pleasant to take a ride than walking in the mud
The truth is that it is safer from two threats sitting on top and they are the biggest threats to armour today.
Threat one is landmines... if you are sitting on a seat in an armoured vehicle and a landmine explodes beneath the vehicle there are a lot of back injuries to crew and passengers. If it is part of an ambush try getting out of an APC with full gear with back pain.
The second thing is HEAT warheads, either from rocket propelled grenades like RPG rounds of ATGMs, which turn the inside of an armoured vehicle into a pressure cooker. The liquid hot gas of the metal liner will destroy lung tissue and burn exposed skin and generally fk you up. Sitting on top you might get light fragment wounds or even be able to jump clear before the rocket hits.
Another reason is that it is stuffy and dusty inside armoured personel carriers.
Look at photos of any APC that is not in direct contact with the enemy and you will see soldiers all round the world do the same, including on M113s in Vietnam etc etc.
Regarding protection from IEDs, if the IED is using a HEAT warhead to penetrate armour then sitting on top is better than sitting inside for the reasons stated above. If however the IED is just a really big bomb to take out the vehicle with blast or fragment damage then it is usually safer inside the vehicle than on top... if the bomb is big enough (like a wired 500kg aerial bomb) then it really doesn't matter whether you are in or out of the vehicle.
What GazB said
Thank you Kvavka. Well, I believe that for similar problems people often find similar solutions, but the two seem very very alike
Go from above because in cases of undermining on a land mine, the chance to survive is much more than if they went inside, all impact will be accepted with the machine, and they only will fly up upwards and them will scatter. Well and in cases of that they can quickly leave the machine if to go inside, longer to leave the machine on time.
Search radar of S-300 missile
What's this russian body armor made of? Ceramic plates?