The article didn't say - it was more of a quick press release.
Of course, it could equally be just as little to do with covid as the vaccine for the same reasons - I hope that, with the wealth of medical, physiological, and performance information held on professional footballers and other athletes, they should be able to dig into the data and isolate any potential trends and causes.
Could actually be a very promising area for a longitudinal study given the sheer amount of high quality data.
Only an opinion, but I wonder if going back into intensive sport after a layoff has caused some problems, like you said in your earlier post.