Steam is generated in your Philips steam generator's soleplate. When you release the steam trigger, any remaining water is still transformed into steam. Therefore, steam is still emitted when you put the iron back, even though you no longer press the steam trigger. As the steam is directed at the stand tray and reflected, it creates the impression of a steam boost. This is normal.
Release the steam trigger 2 to 3 seconds before you place the iron back on the stand tray to avoid this phenomenon.