It happens because nail polish contains solvents. You can actually apply a fresh layer of polish to an old manicure and it will soften it so a few minutes later you can grab a tissue and most of the previously dried layers will pull off. The topcoat is doing that, re-wetting the previously dry layers and dragging the colour onto your brush. A good thick gentle applied floaty layer will help prevent this, as well as working a bit faster. I do find certain brands will do this no matter what and for me personally I save myself a headache and just avoid them. I find thinner topcoats are hard to float as they can just pool. My preference is a nice thick quick drying topcoat like Seche Vite, Posche, HK Girl, Nubar Diamont, etc. I had a few manis done on me years ago and she used OPI topcoat (first to last I believe). It bubbled horrendously on me (a lot of topcoats have done that with me though) and had such a bad finish that I had to remove it immediately and ended up bringing my own bottle of Seche Vite with me. OPI make quality products and I'm sure many techs use them to great effect, but I will use what works best for me and what I can achieve a good glassy finish with. Practise on some pops and wipe off your brush with some kitchen roll, so you don't contaminate your bottle, until you've got it mastered. X