I worked out why colleague was so cool towards me when I started a new project. I thought he just didn't like me. But no, it's not about me. Turns out he was in the midst of job-hunting. He found the new job and is leaving this week.
I've seen this behaviour so many times in my working career but my basic paranoia always makes me think it's personal. It almost never is. Usually it's job-hunting, sometimes it's a sick or dying relative, once it was gambling.
The best time was when I was the IT manager for a company and I was fighting with the CFO who kept blaming me for the financial state of the company. (An independent auditor had blamed the GM and CFO and exonerated me.) But he decided to make me redundant (a 'nice' way of firing me) anyway. I went above his head and asked the GM to intervene but he acted as though it wasn't his problem. Once again my paranoia told me it was about me. But as I was walking to the exit on my last day there, the GM walked out with me. He'd been fired by his boss. The bastard CFO got away clean.