Depending on the flash controller, TRIM will preemptively zero sectors. It can also prompt re-organization of the physical sectors in non SLC drives, both for wear leveling and because the drive will write in SLC mode for speed, but then want to re-pack the data using all available layers, which is a slow operation. As for why it zero sectors, changing a single bit requires zeroing the entire sector, then rewriting it with the bit flipped. The 1->0 transition is slower than the 0->1 transition, and even if it were the same speed, not doing it in advance would cut the speed in half.
The reason you can’t rely on it being a secure delete on any arbitrary drive is the spec doesn’t require the drive to do anything, the firmware is closed source so we can’t tell when it will do something, and even on drives which do actually pre-zero sectors, they wait until the drive is idle to zero and repack data, and there is no way to ask what the status of that process is.
On a related note, SSDs do require power to maintain their state; they are not perfectly non-volatile. The potentials internal to the cells drift over time, which will cause bit-rot if left unpowered for long enough (for most drives, multiple years). While powered up, but idle, the controller will keep track of how long it’s been since a cell was written, and periodically check its potentials, rewriting the data when they drift too far. The energy required for this is tiny, but it is why using SSDs for long term data storage is not a good idea.