What went wrong
Your floor did not break. It shrank. Around the first cold snap the heating comes on, the windows stay shut, and the planks lose a little moisture. Each one pulls back by a hair, and across a long run those hairs add up to a line you can see from standing height.
What makes it feel like damage is the inconsistency. One joint is tight, the next has a gap you can catch a fingernail in, and the worst is where the light rakes across it. That is normal for a floor that is not glued or nailed down.
The mistake, and I have made it, is reaching for a tube of filler. A seasonal gap on a floating floor is meant to be there in February and gone by June. Fill it and the filler either crumbles out as the floor moves, or bridges the joint and pushes the pressure into the next seam. That is how a winter gap becomes a summer peak.
A normal winter gap is narrow, even and level. A gap with a lip, a soft edge or a stain is a repair, not weather.
Why it happened
Dry air. Winter air holds less moisture and heating dries it further, so wood-fibre cores shrink. Many makers suggest 35–50% relative humidity; your carton is the number that counts.
A summer peak that became a winter gap. If the floor was tight to the walls in July, it peaked and the locking edges took the load. Crushed edges do not spring back, so by January the joint sits open. A rough edge is damage, not weather.
One gap instead of many. Seasonal shrinkage shows up across a lot of seams at once. A single wide gap at one end of a run, with everything else tight, usually means a plank was never fully clicked.
Water, or a subfloor that moves. A gap with a soft edge, a tide mark, a musty smell or a squeak beside it is not a humidity story. Check for moisture and for a loose subfloor with a squeak hunt before anything cosmetic.
Floor type changes the answer. Laminate moves the most because its core is wood fibre. LVP moves with temperature more than humidity, so a winter gap in vinyl is usually a bad click. Solid hardwood loses width in dry heat and closes in summer.
When to stop and call someone
Stop if the gap has a lip, if a plank rocks underfoot, or if it keeps widening after the room is back in its humidity range. Stop if the edge feels soft or there is a stain, because that is moisture, not weather. And stop if the floor is glued down or nailed: cutting a gap in a fixed floor is a different job with different risks.