counter offers for a company under a t/o offer usually come from...

counter offers for a company under a t/o offer usually come from companies that don't necessarily want to buy the t/o target but more importantly, don't want the bidding company to acquire the target. this is why counter bidders only get flushed out when a company makes an initial bid. the counter bidder is usually doing it for strategic reasons and not because they necessarily feel there is any great value in the target company.
I don't think a t/o by tullow is that a strategic consideration for other companies to justify a counter offer for a company that they could've acquired 6 weeks ago for much less