If I (as a defender) intercept a pass from the offense, and I am in-bounds, can I drop the disc to the ground so my teammate pick it up, becoming the thrower and resuming the play?
You cannot intentionally drop the disc so that a teammate can take possession, as stated in 13.6:
If the player in possession after a turnover intentionally drops the disc, or places the disc on the ground, they must re-establish possession.
When it says “they must re-establish possession” it is referring to “the player in possession after a turnover”.
However if you never have possession (ie you try to catch it but fail, or are ground stripped) then your teammate can take possession.
what was the reasoning behind this change?
The majority of times this occurred in the past was due to ignorance of the rules.
Therefore it was adjusted to be more in line with rule 1.2:
It is trusted that no player will intentionally break the rules; thus there are no harsh penalties for breaches, but rather a method for resuming play in a manner which simulates what would most likely have occurred had there been no breach.