I suspect the reason to be that the system has noticed that your apps haven't touched their memory for a long time and thus "don't need it". This is a reasonably valid assumption on servers: if you've got some daemons that are backgrounded for minutes or hours at a time, keeping their memory resident is a waste. However, on the desktop responsiveness (latency) is more important than throughput. Just because you only switch between apps on a timescale of minutes or hours, doesn't mean the kernel should swap them out. So the algorithm needs different weighting.

I used to experience this problem, but I haven't lately. I suspect what's going on is that the "-desktop" kernel variant of OpenSUSE uses a differently weighted swappiness algorithm. If your distro offers a choice of different kernel variants, you could try them; otherwise (or if that doesn't help), you could track down the knobs you need to tweak to make the problem go away.