Temperature changes could have caused the bark to split...... e.g.A hard frost at night during the spring whilst the sap is rising.
You didn't say for how many years the position has prevailed. Nevertheless, if it's still flowering, I wouldn't be too worried. I once lit a bonfire a little too near a silver birch tree, about 20 yrs ago and the trunk split. The tree survived until it became far too tall and only a few weeks ago I had it felled. Also I still have a Japanese Maple which must be about 40yrs of age and, whilst that split in it's very early days, I would defy anyone to point out the area of 'scar tissue'.
Ron.