I have been on the other end twice, losing a good friend without knowing why. The first time she very sadly had a baby with a syndrome that meant he only lived a few weeks. She was naturally absolutely distraught with grief, and I tried to support her in every way I could, but she stopped answering emails or phone calls. Eventually DH spoke to her husband, and he said one comment I had made was deemed not to understand her situation, so she wanted no more contact. I could appreciate that this was probably the anger stage of grief, but she never accepted any more contact, and we had been so close.
The second one was an old work colleage, we had been in touch for 19 years after leaving the company we worked for, where we had both been mistreated. I completely forgot a lunch date, I work on Wednesday mornings and had gone home on automatic pilot, my calendar had failed to remind me on my phone. A few months later she forgot a lunch date with me, so it wasn't entirely me. Very regrettably a couple of years later I forgot another one, same circumstances, and when she rang I asked her to to hang on, I would be there in five minutes, but she was too annoyed, said she was going home. She has refused all contact ever since, so I have given up trying.
I only mention those, because a bit like like grandparents cut off from grandchildren, losing friends that have been a part of your life for many years really does hurt, and I think if you have decided someone's behaviour has been so dreadful that you want to cut contact, then in all fairness, you should give them an explanation.