I'm not a great "technician" myself, but it must be because you kept the F-amount but lowered the speed and the amount of light that got thru is higher.
A tip, if available in your camera, switch the live view on and like this you are able to preview live, what the results should be.
If you want to use longer exposures you might consider increasing the f-stop so that in combination with the lower speed the images doesn't get over exposed.
Or, buying those filters that allow "monster exposures" even in broad daylight, it all depends on what you want to achieve with those filters.
Hope this helps in some way.