Trading basis, page-9

Say you bought 1000 shares and immediately accepted the offer and sold 500 to Crescent. Then you sell the other 500 on the market. The person who buys them then immediately sells 250 to Crescent and sells the rest, and so on.

So to stop this happening, if you accept the offer your remaining half are locked until everything has finished. Otherwise Crescent could technically buy more than the 60% via people accepting the offer. So I'd guess that rule was put in place to stop some other rule being broken and would probably be hidden int he T&C's somewhere.