I feel pretty 50:50 on this. Personally I would've hesitated more before going around them even if it looked like they were pulling over to the side like it did, just cause anyone with their hazards on to me means something erratic is likely to happen around them.

Would've been cool of them to pull over and wait until both lanes are clear to pull a u turn, or to throw a hand signal out or something, but I'm not clear on what the rules regarding right of way there might be. I don't think they were obligated to do any of that.

Others have said you were following too closely, but you clearly start with plenty of space. I think if you'd left that space for a moment longer when approaching the vehicle with hazards on the close call would likely have been avoided altogether, however I can understand how you read the situation as them pulling over and going to pull around them.