There could be several reasons for this.
Wireless Charging Heat: Wireless charging itself can generate heat, which is a normal occurrence. However, if the heat becomes excessive, it might indicate a compatibility issue.
Intensive Usage: If you're using your iPhone while it's on the charging stand, the device's processor can generate heat too, causing the device to become warmer.
Faulty Charging Stand or Cable: If the charging stand or the cable you're using is faulty or not of high quality, it could lead to inefficient charging or excessive heat generation.
This said, it is vital to use a high-quality charging stand from reputable manufacturers like Anker to ensure compatibility and smooth operation.