Steam is generated in your Philips steam iron's soleplate. When you release the steam trigger, any remaining water is still transformed into steam. Therefore steam still comes out even though you are not pressing 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 two to three seconds before you place the iron back on the stand tray.