Why indexing dropped in big numbers?

Hi everyone,
I really don't know this topic was opened recently by someone or not! But at this time I have been facing an issue about Google webmaster indexing, I have been working for a industrial client website, today when I login to webmaster I saw indexing of website down in big number, I have also put a snapshot, I want to know the reason of drop indexing so I can suggest recommendation to my client, I observe few of products was removed by client from the website , but before two weeks ago website indexing show 486,660 & now it's 204,515.
As you know the indexing has been dropped in big numbers so I wanted to know reason if someone expert has idea then kindly let me know.
Thanks in advance!

It happens, how old is your website. Larger websites usually suffer indexing problems. Try to make each of your page unique to the others and build links to them.

How old is the site and how long has the index been at 480k?

Someone must have played with the canonical settings or if it's a new site with a bunch of auto-generated content, it's completely natural to lose the numbers.

It's impossible to say without having more details. From first looks it seems like your site had a ton of auto generated content pushed at some point?

The site is 12 years old & related to abrasive industry! I have been handling site since July 2013, probably indexing dropped from last two weeks, We have not used the canonical for any page & also I have own SEO writer who working on new products & blog posting, Also my client have premium copyscape membership so we are publishing content on blog & on products which cross copyscape!
I really not understood why indexing going drop in big numbers.

We are not publishing any auto generated content, but have published the unique content for all products, actually client site is related to abrasive industry & we have published the up to 700K products on the website! the ranking of site still improving day by day, but the client ask me the question about indexing drop which really confused me!