Steam is generated in your Philips steam iron's soleplate. When you release the steam trigger, remaining water is still transformed into steam. Therefore steam still comes out, even though you do not press the steam trigger anymore.
Also, as the steam is directed to the stand tray and reflected, in some cases it looks like a steam boost is created.
To avoid this, please release the steam trigger 2 to 3 seconds before you place the iron back on the stand tray.