why does my ex stare but not communicate?

we dated for 18 month and were on the verge of getting a house together and then after a daft fall out he finished things with me.

while when we first split we initially worked together for 2 weeks he ignored me and walked out off rooms then suddenly needed to b where ever I was. I left this work place and the only contact we had was via text arguing over money he owed me. whilst arguing over this he accused me off seeing someone else. Things got nasty between us and he deleted me off a social site.

W had a night out int he same group of friends and he refused a drink off me and never spoke to me all night, when most of the group had went home and there was only myself him and my friends out he was staring and watching me dance and then when he realized id noticed he looked at the floor then after doing this several time he left.

I stopped contacting him and have had no contact with him for over 4 weeks and recently called into my old work place to pick things up and found he just stared at me, I then passed him on the road driving and 10 mins later had a missed call of him yet when I txt asking why he called he never answered?

Why is he doing this?