Wind direction does matter! Just to make that clear from the start.
Harare Airport is located at an elevation of 1490m MSL, and Zimbabwe is in tropical region, which classify the Airport in hot and high conditions. That's also why Harare has one of the longest runways in Africa.
As I understand it, pilots are required to follow standard procedures, but on some specific conditions, they are able to submit a request for special ones (or suggested by controllers). To list a few :
- start take off roll from runway entry instead of from one end of runway (to save time when late on schedule, to ease traffic flow, to avoid a runway backtrack when no taxiway leads to runway start while an aircraft is on an approach pattern, etc.)
- downwind take off (similar to above, or last take off(s) after a wind change, etc.)
- take off from the runway that shortens flightplan length/duration (there are specific paths you must follow (waypoints) to exit the area of an airport, and reach the air routes toward destination. There are also forbidden areas like cities or military/restricted zones you have to avoid). Obviously, taking off direction matters.
- on a calm day with clear visibility and nearly no wind, on an airport with not that much traffic, you have much more opportunity to select a preferred runway depending on the flightplan.
There are also some cases where you can land from one runway end and take off from the other. I don't know whether Harare suits those cases, but there are several airports in the world which are. Obviously the ones that have closed runway on one end like Lukla (VNLK) or Courchevel, but you also have Nosy Be (FMNN) and Antsiranana (FMNA) in Madagascar which aren't, to name those I know well, and another one that is closed: Kai Tak (VHHH) in Hong Kong. Reasons vary:
- Terrain. When you have mountains right in the take off path.
- Winds, likely to come from one side along the year, that you can neglect on landing on calm days, but not on take off.
- Lack of Navigation support from one end of the runway (ILS/PAPI/VASI/VOR/DME...)
- Restricted area (City, Military bases, highly secured facilities, etc.)
- Gates/parking spots location: when they are near the end of one runway, take offs are likely to start there, and landing to occur from the other end, when possible.
- noise abatement : landings can be tolerated from one end, but not take offs.
Now, sum the above for Harare:
- you have such a long runway that sometimes, not all aircraft are required to take off from one end. Reciprocally, you have to taxi longer to reach Rwy23 for take of, and taxi back longer to reach the terminal if you landed on Rwy05.
- you have many airlines operating there, but there is not that much activity as on platforms like JFK, Manchester, Singapore Changi or Sydney.
- you have airliners, commuters, general aviation and military; large, medium and small aircraft. Depending on their flightplans and the weather condition, you can expect to have take offs and landings from both directions, anytime.
- you're in Africa, where rules are less strict than anywhere else (good or bad ? I don't know)
So, when winds are calm, visibility clear enough, runway long enough for a given aircraft to take off from a specific point, you can expect pilots to select the best runway that fits their flightplans. This does not happen only in Harare, it happens on a daily basis, like on my home airport as well, Antananarivo Ivato (FMMI/TNR):
Due to winds direction mostly coming from east, take off are expected to occur from Rwy11. But, small airliners like ATR or some 737/ERJs doesn't backtrack Rwy11 at all. Only "visitors" backtrack the runway. Medevac small aircraft (Cessna 206B, Learjet 31, King Air) usually land from the runway that suits their flightplans. A few minutes later, a DHC6 takes off downwind (Rwy29) because the destination is to the west. Then a 777F of Air France lands on Rwy11. And so on while the terminal is right in the middle between the two ends of the runway, and it's only 3100m long.
By the way, pilots or controllers don't just do what they want. Every decision is taken with the uttermost professionalism and care, and whenever that is required, this is registered in log books, or a clearance is requested, and granted beforehand. That means almost all, if not all, activity you may spot on Harare are known and defined procedures. It's highly unlikely a pilot would perform a procedure not written anywhere (the agency/organization in charge of navigation safety in the country is responsible for providing standard procedures).