A "long squeeze" is a bearish scenario where a high percentage of traders are long in a currency pair when the market begins to move against them.
Since most traders use stop-loss entry orders to limit losses or will close out positions if they move against them too much long squeezes can accelerate as stops are tripped to cover losses. Also since most of the traders are already long their buying power may be limited as the pair moves against them, further accelerating the bearish move. For example, lets say 80% of speculative traders are long the EUR/USD pair. If the pair starts moving downward on heavy bank selling then two things happen: a) traders are unable to buy further as they are already long, and b) as their stop-loss orders get tripped the pair will accelerate downward.
Also see short squeeze.