Built house next to lava pit, covered entire pit with dirt, walked away for a few minutes and came back and part of my house was burned down

I built a house next to a lava pit but made sure to cover the entire pit with dirt so it wouldn't burn my house down. Afterwards I went exploring a nearby cave and when I came back part of my house burned down.

What I expected to happen was, after I covered the lava pit with dirt, that it wouldn't burn my house down

What actually happened was it burned my house down

1. Build a house next to a lava pit.
2. Cover the pit with dirt.
3. Walk 100 or more blocks away for 15-30 minutes and come back.