Not very confident, but given that several what appears to be independent mutations have been identified, but with mutations in similar regions and at a similar time, it could make sense that a new or increasingly used therapy form could be behind this.
I have seen articles in the National Geographic and Indian Express suggesting that some of the newer, more infectious covid strains that have been popping up lately may have originated from patients treated with convalescent plasma therapy.
I haven't been able to find much information on this, except for a preprint on medrXiv[1] about an immunocompromised patient with a prolonged SARS-CoV-2 infection had his viral DNA sequenced repeatedly over a 101 day period. After treatments with convalescent plasma, there were significant changes in the viral spike proteins with were associated with reduced susceptibility to antibodies and potentially greater infectiousness.
Neutralising antibodies in Spike mediated SARS-CoV-2 adaptation, SA Kemp et al.](https://doi.org/10.1101/2020.12.05.20241927) ↩︎