Steam is generated in your Philips steam generator's soleplate. When you release the steam trigger, remaining water is still transformed into steam. Therefore steam still comes out when you put the iron back, even though you do not press the steam trigger anymore. As the steam is directed at the stand tray and reflected, the impression of getting a steam boost is created. 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.