WHY? Well... Because they are (or were) in trouble with WAY too many silver orders to deliver or roll over or settle for cash..... or something.
So what did they do? They declared a 'technical glitch' that stopped trading for 90 minutes. During that time, they took out a big big chunk of standing orders ('open interest') and settled them for cash or something else... 
They disappeared, basically, while the trading was off line. 
They had to do this because the vaults are running low on silver.
This is totally obvious scam and fraud. It is amazing that this is allowed to happen. It appears the COMEX is basically fake silver trading operation where for the most part, no (or little) silver is ever delivered - and the COMEX itself sees to it that it never happens. 
The good news is that last time they did this (in November) silver prices subsequently went up by 150%. 
There is reason to believe that the days of silver paper trading are coming to an end. This cannot continue without serious credibility issues.
Not investment advice. Do your own research - but do it soon!