Water flowing backwards from pool to spillover spa

[niting]

Hi experts, I opened my salt water pool today and everything went fine. However, as the water started circulating, I saw something weird. Instead of water flowing from spa to the pool as has been the case all these years, the water was flowing from pool to spa. Ofcourse it is bringing all the debris with it to the spa and the pool skimmers are not pulling anything. The water flow works fine in the spa mode.

This is a head scratcher. Any ideas why this may be happening? Thank you.