Is it me???

I've gotten myself into a bit of a situation and I'd like advice about it, please. I guess it all started about two weeks ago when i was mowing my front yard. It was a beautiful day, slight cool breeze and I felt great. I wasn't having any fatigue during mowing like I usually do. It was easy to do yard work that day and I was kinda having fun. I finished mowing my yard and still had time and energy to burn, so I decided to mow my neighbor's front yard as well. It was overgrown and standing about a foot and a half tall. My neighbors have been my friends for over 6 years and we often do things for each other. A loan of a shovel here, a borrowing of sugar there, a kind of a mutual "I help you out, you help me out" relationship. I really didn't think too much about mowing their yard, especially since one of them works all day and the other has been sick for quite awhile. I did not expect, nor ask for compensation. I was only trying to help out. I mowed and cleaned up the grass debris, being sure to treat their yard with the same amount of care that I do my own. Afterwards I get a text telling me that they didn't need my "help" and were going to mow it themselves later that day. (Odd because they hadn't mowed it in months, nor did they since mow their back yard, but whatever) They told me to "stay off their property!"

I was a little hurt by the ingraditude but I understood. I didn't ask for permission, as I didn't think it would be a problem. But that's my mistake, I see how I overstepped boundaries, so I apologized and reassured that I would never again mow their yard. But my apology wasn't received well at all. They act as though I was maliciously on their property. They have surveillance cameras so the could easily watch exactly what I did and see that, indeed, there was no ill intent nor activity. But that doesn't matter to them. The next day I walked out my front door at the same time they were outside and immediately heard, "Here she comes, don't look! Don't look!" Baffelled I watched as they ducked inside their house as if dodging a stalker or something. I thought, "that was strange" and carried on with my day. Days go by without seeing them, or getting the usual "hey, how are ya?" texts. Then yesterday my son and I were outside playing ball in the yard like always and the ball goes into their yard, I run to get it and suddenly the front door opens and I'm greeted with a loud, "heyyy! Stay off our f*cking property, bitch!!!" It seems as though our friendship is over and they are trying to incite some sort of neighborly argument over the mowing. I get it, I mowed without their permission, but I did not do so in malice and I apologized. We've been close friends for 6 years, even spending last Christmas celebrating and gift giving together. I just don't understand. Is it me?? Did I grossly insult them or violate neighborly code of conduct so egregiously that this treatment is appropriate?? I mean, I get it, I mowed without their permission, but I meant no harm and apologized. Is this really worth throwing away our friendship??? Is it me or is this over dramatic? I kinda think maybe there is other factors at play outside of my involvement. They both are bipolar, not to excuse my actions with their diagnosis, but I have been considering whether the bipolar is motivating their reactions. Please tell me what you think and advice on how to move forward positively, please. Thank you ever so for listening.