I see the ranking of Leeds MBA was quite ok ( 78th in 2018) but their ranking dropped so much, recently the program is out Of top 100 in ft.

What is the key issues there?

Nothing special about Leeds. UK government policies like the "hostile environment" policy and Brexit reduce the demand for foreign talent, especially from mid-quality MBAs that don't require the GMAT. 

A lot of schools in that tier dropped a lot, and many are no longer in the ranking anymore. They were 92nd in the 2018 FT Global MBA Ranking by the way. Look at other UK schools in that range, Lancaster, Strathclyde, Bath, all out of the ranking now. Edinburgh's not too far behind.

A major issue is that these schools recruit international candidates (who may or may not take the GMAT as Duncan mentioned) who don't really have the qualities that are sought after in the UK labor market. So they end up going back to their home countries, making lower salaries that drag down the ranking. 

Nothing too surprising here, honestly.