There are 3 reasons this may be happening, lionwaffles and lord Jorge touched on 2 of them.
- They may have accidently dropped a troop when trying to click next or end battle.
The reason they would then end the battle is possibly because they don't want to waste their troops trying to get the amount of loot you have, or they don't think their troops are strong enough to successfully attack you. In both cases they decided losing trophies was better than wasting troops.
- They may be actively trying to drop trophies.
To expand on why someone would want to drop trophies, they may have pushed trophies to get to a certain league or trophy count for a gem reward and wish to return to their lower league, they may be approaching the trophy count to get promoted to the next league and want to drop trophies to remain in their current league, they may want to drop to a lower league to find easier bases (generally speaking the higher the trophy level, the stronger the bases are), or they may want to drop trophies to protect loot (at higher trophies people will generally attack with stronger armies).
- They may have actually been trying to raid your base and accidently clicked end battle when trying to switch or deploy troops.