I think you have highlighted the issue GBG has with price...

I think you have highlighted the issue GBG has with price stability at the moment. SDL was heavily margined pre the takeover failure news and would have created margin calls post announcement. If you wanted to clean up your margin event then its logical to sell shares in your other holdings to cover. Especially if selling a called share like SDL doesnt cover the margin dept.
GBG shares have had huge turnover and price reduction since the SDL announcement as well.
This company das been hit by a freak chain of events in the last 2 months. Hopefully this will change soon.