Bubbles in hydrometer sample

My Kolsch has been in primary for 4 weeks now (fermented with WLP029) at 62 degrees) and the gravity has been steady at 1.008 for the last week.

But when I took my most recent hydrometer sample, it was full of bubbles! The hydrometer was covered in them, and so many new ones were forming that I couldn't clear it just by spinning. I had to let it sit for 10 minutes before I could get an accurate reading.

I assume this is just existing CO2 degassing from the beer, rather than a sign of ongoing fermentation, but it surprised me since I've never seen so many bubbles in any of my previous brews.

Anyone able to shed some light on why this might be the case? Why would this particular beer have so much more CO2 left in it than I have ever seen before? It is my first time using Kolsch yeast, so maybe that's the difference?

It's difficult to say - it is almost certainly CO2 coming out of solution in response to agitation and/or temperature change. Did you maybe store this beer colder than you usually do? That would explain the higher amount of CO2 compared to beer stored at ambient temperature.