Hmm this is one that actually falls under my area of expertise for once (I’m normally a pure knowledge leech on these boards)
its possible the building had a combustible external facade. these are normally referred to as EIFS - exterior insulation and finish system. as noted above, these are often a plastic - typically polystyrene (EPS) or polyurethane (PUF/PIF) sandwiched between aluminium or steel. All plastics are combustible and burn aggressively. Even the ‘fire safe’ plastics still burn, they are just less likely to propagate the fire in and of themselves.
We recently suffered a major fire in a melbourne building due to use of this type of EIFS (check it out http://www.theage.com.au/victoria/docklands-apartment-tower-fire-fuelled-by-material-in-buildings-walls-says-mfb-20150427-1mukhx.html ) As a result its use is being disallowed throughout all of Australia and many facilities are retrospectively removing it (and thats big $$$)
however even in a traditional glass and concrete structure, fire can easily propagate to the extent shown. the scenario is this - uncontrolled fire on one floor, involving standard office or hotel furnishings (carpet, chairs, desks, curtains, trims, wooden furniture). Without sprinkler control, the fire will get so hot the windows blow out. this means the fire will now lap up the outside of the buildings. it will damage next floor up windows, and blow them in. Then set all combustibles on floor n+1 on fire. now the flame front outside the building is being magnified by 2 floors spewing fire - the total flame height is now larger and stronger (imagine holding two lighter flames together - they greatly increase). It then sets n+2 on fire, and so on. At the level 20, the brigade cannot effectively control the fire from the ground so they must do it internally which is slow, dangerous, and ineffective.
sprinklers are the only real way to control this sort of fire starting like that unless you catch it so early an individual snuffs it out.
additionally having sufficient window spacing can limit or prevent external fire spread - you need ~50% window per the building facade. this isnt so attractive and nearly all buildings have 90-100% glass facade nowadays