There's a chemical reaction between the metals, you're messing that up when you move the wires. If you want to reduce this problem, you can try switching to 63/37 solder, which goes transitions much faster to solid, at a fixed temperature. The 60/40 has a wider temperature range, so you have to keep the 2 parts fixed until the solder goes outside that transition temperature range and becomes solid.
There's a very good series of soldering tutorials at Pace's youtube channel, here's the first part:
The other parts are in the same channel, easy to find.
The videos are a bit dated, but the information still applies to this day and the production quality is quite good.