Why does the effective alpha/CL increase at wingtip of swept wing?
You are reading the wrong sources.

I've never come across the claim that the lift curve slope of a swept wing increases towards the tip. That might happen in ground effect (when the tips are very close to the ground from the swept wing's incidence), but that effect is very weak, or when the wing tip has noticeably less sweep (crescent wing).

But not on "regular" swept wings in normal operation.

If, however, the intended meaning of your question is to ask why the tips of backward swept wings reach stall conditions earlier than the inner wing: That they do indeed. Read on for an explanation.

What your sources describe is foremost caused by the thickening of the outer boundary layer of backward swept wings. This is compounded by aggressive taper - but this taper effect is very similar in unswept wings, too. Next, when you plot aspect ratio over sweep angle, you will find a boundary above which swept wings develop undesirable stall behavior, so the wing's aspect ratio is another factor (which your sources stay mum about).

To prepare for the interview, I advise you to read some of the answers here, like:


Swept wing layout for maximum lift at transsonic Mach numbers.
Sub- and transsonic flow separation.
Stability of swept wings.
Why the better designs use increased sweep at the tip, precisely to lower the local lift curve slope.


And make sure you follow the links in the linked answers as well if something is unclear.