Some are still working, although I doubt that'll be a 5 minute job. Have to wonder if the reinforcing in the lintel has corroded, causing it to crack initially, seems sturdy enough otherwise to bridge that gap.
There doesn't appear to be any downward thrust from the masonry above, so the failure looks to be from within the concrete lintel, as suggested - rusted rebar etc.
Looks like a "classic" case of the embedded reinforcing bar, or bars rusting, in doing so the metal as it deteriorates expands, the concrete then MUST crack,
Generally the horizontal crack is lower down on the lintel?
What age is the property? between WW1 + WW2 or just after WW2 at that time the [so called] cover, the distance between the outer surface of the concrete lintel and the reinforcing bar was not clearly understood, this resulted in rain water getting in to the concrete lintel and the corrosion [rust] is the result
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