I replaced radiator in my 2006 cadillac cts and the return line for the transmission popped out of the rad.... why?

If water got into the transmission, there is no telling how much damage it causeed to the pumps, torque convertor and/or clutches. I am sorry to say you are probably in for a transmission rebuild or replacement.

This is a very easy job if you are mechanically inclined. Remove rad cap, remove the lower rad hose to let coolant drain out. Catch in a clean container to dispose off properly - do not put into drain.
Disconnect transmission lines using line wrench to preventing rounding out of nuts. Remove top rad hose and any top plate holding the rad in position. gently wriggle and lift out of the car.

if you car have the electric fans mounted onto the rad = transfer over to new rad without breaking any mounting bolts.

You may as well put in new coolant at this time than re-using the old coolant.

Who did the work? With that much work done it should have come with some kind of warranty. I would take your car back to the person that did the work and ask them to take a look at it. Could be a fuel problem (fuel filter, pump, feed or return lines) or it could be a calibration that needs to be adjusted in your on-board computer. I would start with running a full diagnostic from a reputable company and see if it shows anything.

Drop the transmission oil pan, replace filter, reinstall pan. Remove inspection cover to view torque converter. Turn flywheel over by hand till drain plug in converter is at bottom. Remove plug and drain. Replace plug and fill trans. with new fluid.

Your transmission cooler line may be bad at the input side into the radiator. I had a similar problem on my truck and all I needed to do was tighten the fitting.

Check and replace fuel filter if not done in last 50k, clear codes and recheck.

Sounds like you have a bad radiator, Get both flushed and replace radiator.

replace your radator there is a trans cooler on the side of the rad the leak is internal

tod34 is correct. The "fill" area of this transmission is located on the transmission itself. It is a fluid leveling plug located low on the main case toward the back of the trans. You must pump the fluid in through that hole. If you are on level ground and the transmission is properly warmed, you should fill the transmission until the fluid level is flush with the bottom of the hole (starts spilling out)

DOES'NT MATTER WHAT YOU OWN FOR A CAR THE THERMOSTAT ALWAYS IS FOUND IN THE UPPER RAD HOSE,IT WILL TAKE YOU THE THERMOSTAT HOUSEING.REMOVE THE HOUSEING AND INSTALL THE NEW ONE[AFTER YOU HAVE DRAINED DOWN THE SYSTEM].